Skipping over 2.4.0 to sidestep the `"symbol_from_string".to_sym.dup` bug.

References #32028

Rails 6 will only support Ruby >= 2.3.

I'm not sure cause, but due to the influence of `gem clean`, the expected
gem seems not to be installed correctly.
In order to avoid a test failure due to this, I fixed that `gem clean` not be executed.
Ref: https://github.com/travis-ci/travis-ci/issues/2518#issuecomment-121168856

This is a workaround. If Travis fixes something, please revert this.

  RubyGems 2.7 promote bundler-1.16 to default gems.
  It conflicts Travis environment and user installed bundler.

This reverts commit 6ec0ed67.

JRuby 9.1.15.0 has been released:
http://jruby.org/2017/12/07/jruby-9-1-15-0.html

These are needed when GEM=ast, thanks @georgeclaghorn

JRuby 9.1.14.0 has been released and this version is available on Travis CI.
http://jruby.org/2017/11/08/jruby-9-1-14-0

Since 8f2490b5, the integration test of sidekiq and resque is not working
in CI.
https://travis-ci.org/rails/rails/jobs/301276197#L2055
https://travis-ci.org/rails/rails/jobs/301276197#L2061

Because 8f2490b5 removed password from `redis-server`.
So must also remove passwords from these tests.

Simplify our dev testing and CI story since we're also testing against
Redis for the Active Support cache store.

Directly test whether db, host, password, etc are passed through as
config instead of spinning up a Redis server with a password set on it.

This reverts commit 6f481e05, reversing
changes made to 592f790b.

In favor of #31039.

when tested with ruby-head

This pull request attempts to ignore the following error
when tested with ruby-head which has bundler as a default gem.

```ruby
$ rvm @global do gem uninstall bundler --all --ignore-dependencies --executables
ERROR:  While executing gem ... (Gem::InstallError)

    gem "bundler" cannot be uninstalled because it is a default gem

The command "rvm @global do gem uninstall bundler --all --ignore-dependencies --executables" failed and exited with 1 during .
```

Refer https://travis-ci.org/rails/rails/jobs/295600391

This workaround should be removed once https://github.com/bundler/bundler/issues/6072 is addressed.

Since 1.16.0 is installed by default, it seems that the newer one will be used
even if specify an older version.
Ref: https://travis-ci.org/rails/rails/jobs/295553738#L1718

Follow up of #31023

It's causing bug report template tests to fail. https://travis-ci.org/rails/rails/jobs/295520851
This seems an issue of bundler. Ref: https://github.com/bundler/bundler/issues/6072

Most interesting to us, 2.4.2 and 2.3.5 include the fix for
https://bugs.ruby-lang.org/issues/13632

http://jruby.org/2017/09/06/jruby-9-1-13-0.html

Travis CI new default Ubuntu Trusty supports 9.6
https://docs.travis-ci.com/user/database-setup/#Using-a-different-PostgreSQL-Version

- Travis CI is migrating Ubuntu version to Trusty
- MariaDB 10.2 is supported on Ubuntu Trusty

The value isn't actually secret, so I've kept it as a comment.

- Adds RedisAdapterTest::AlternateConfiguration to account
  for a relatively common alternative setup, as it’s used
  as the first example in the
  [Redis rubygem](https://github.com/redis/redis-rb#getting-started)

- Supplies original RedisAdapterTest with more complete
  redis:// url format by adding a ‘userinfo’ (blank user),
  so that it resembles the alternate configuration

- Supplies original EventedRedisAdapterTest with more complete
  redis:// url as well

- Adds before_script to start redis-server with password as a daemon
  and with explicit defaults copied from the default redis.conf
  (Instead of using Travis' default init/upstart scripts for `redis` service)

http://jruby.org/2017/06/15/jruby-9-1-12-0.html

http://jruby.org/2017/05/25/jruby-9-1-10-0.html

This version lock added by 3d890b66.
But original issue is fixed with bundler 1.15.

Ref: https://github.com/rubygems/rubygems/issues/1911#issuecomment-300148516

http://jruby.org/2017/05/16/jruby-9-1-9-0

It's causing a test to fail, and we're not yet sure what (if anything)
we should be doing differently. See rubygems/rubygems#1911.

Revert "Merge pull request #27636 from mtsmfm/disable-referential-integrity-without-superuser-privilege-take-2"

This reverts commit c1faca63, reversing
changes made to 8c658a0e.

See https://github.com/rails/rails/pull/27636#issuecomment-297534129

Also, explicitly apply the order: generate_subscripts is unlikely to
start returning values out of order, but we should still be clear about
what we want.