Fixes #32129.

Although not a direct dependency, it's pulled in by Resque for
Active Job integration tests so we need to update because the
rack-protection gem has a security vulnerability[1].

[1]: https://nvd.nist.gov/vuln/detail/CVE-2018-7212

Refer to a stable release of SDoc

We locked Minitest to 5.11.1 in #31799
because 5.11.2 included a breaking change.
The change was fixed in 5.11.3, so we no
longer need to lock in the version.

To workaround `undefined method `error?' for` reported
at https://travis-ci.org/rails/rails/jobs/333456146

Follow up to e1473e0c

This will avoid the branch being deleted by mistake making all builds to
fail.

Queue_classic currently limits pg to "< 0.20".
It is therefore not used for rails CI tests. There has been
a bull request for a while (by a co-worker of mine), which
fixes the incompatibilities and extends dependencies to pg-1.x.

This patch add this pull request to the Gemfile as an interim
solution, until it is merged.

Seems some tests not work with Minitest 5.11.0.

* https://travis-ci.org/rails/rails/jobs/323997512#L1053
* https://travis-ci.org/rails/rails/jobs/323997486#L1055

Ref: https://github.com/seattlerb/minitest/issues/729

Ref: https://github.com/collectiveidea/delayed_job/blob/master/CHANGELOG.md#414---2017-12-29

For pass Active Job integration tests with Ruby 2.5.
Ref: https://github.com/collectiveidea/delayed_job/pull/996

The v4.3.1 has already released that includes Redis 4.0 support.
https://github.com/resque/resque-scheduler/blob/master/CHANGELOG.md#431---2017-11-20

The `keyfile` was renamed to `credentials` in `google-cloud-storage` 1.8.0.
https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/master/google-cloud-storage/CHANGELOG.md#180--2017-11-14

Although `keyfile` can still be used, but it looks like deprecate.
https://github.com/GoogleCloudPlatform/google-cloud-ruby/blob/ddf7b2a856d676316525eb581c1a4cc83ca6097b/google-cloud-storage/lib/google/cloud/storage.rb#L589...L590

Therefore, I think that should use `credentials` in newly generated
applications.

Ref: https://github.com/GoogleCloudPlatform/google-cloud-ruby/issues/1802

`em-hiredis` is unused since 48766e32

https://travis-ci.org/rails/rails/jobs/304428814#L1977

Move back to resque-scheduler mainline now that https://github.com/resque/resque-scheduler/pull/620 is merged

## Summary

RuboCop 0.51.0 was released.
https://github.com/bbatsov/rubocop/releases/tag/v0.51.0

And rubocop-0-51 channel is available in Code Climate.
https://github.com/codeclimate/codeclimate-rubocop/issues/109

This PR will bump RuboCop to 0.51.0 and fixes the following new
offenses.

```console
% bundle exec rubocop
Inspecting 2358 files

(snip)

Offenses:

actionpack/lib/action_controller/metal/http_authentication.rb:251:59: C:
Prefer double-quoted strings unless you need single quotes to avoid
extra backslashes for escaping.
          [key.strip, value.to_s.gsub(/^"|"$/, "").delete('\'')]
                                                          ^^^^
activesupport/test/core_ext/load_error_test.rb:8:39: C: Prefer
double-quoted strings unless you need single quotes to avoid extra
backslashes for escaping.
    assert_raise(LoadError) { require 'no_this_file_don\'t_exist' }
                                      ^^^^^^^^^^^^^^^^^^^^^^^^^^^

2358 files inspected, 2 offenses detected
```

The `redis-namespace` 1.6.0 includes redis-rb 4.0 support.

Reverts 4d96be1c
References #31026

```ruby
TestHelperMailerTest#test_encode
BaseTest#test_implicit_multipart_with_attachments_creates_nested_parts
BaseTest#test_implicit_multipart_with_attachments_and_sort_order
BaseTest#test_explicit_multipart_with_attachments_creates_nested_parts
```
Refer https://travis-ci.org/rails/rails/jobs/295571582

https://travis-ci.org/rails/rails/jobs/295470534#L2133

"The dependency activerecord-jdbcsqlite3-adapter (>= 1.3.0) will be unused by any of the platforms Bundler is installing for. Bundler is installing for ruby, x64-mingw32, x86-mingw32 but the dependency is only for java. To add those platforms to the bundle, run `bundle lock --add-platform java`."

This has been added by 8f8cb1ba.
But now it is unnecessary because it is not used in the test.

RuboCop 0.50.0 was released.
https://github.com/bbatsov/rubocop/releases/tag/v0.50.0

And `rubocop-0-50` channel is available in Code Climate.
https://github.com/codeclimate/codeclimate-rubocop/issues/107#issuecomment-336234260

This commit will bump RuboCop to 0.50.0.
There are no new offences in this change.

```console
% bundle exec rubocop --version
0.50.0
% bundle exec rubocop
Inspecting 2350 files

(snip)

2350 files inspected, no offenses detected
```

The sidekiq 5.0.5 includes redis-rb 4.0 support.
Ref: https://github.com/mperham/sidekiq/blob/90db3b84208cbb73a50d1a77a1dea97d3490ce70/Changes.md#505