Fixed example of `Rails.application.encrypted` method usage

[ci skip]

We generate master key on `rails new`.
Therefore, if do not add master key to `.gitginore` on `rails new`as
well, there is a possibility that the master key will be committed
accidentally.

Fixes https://github.com/rails/rails/pull/30940

To edit/show encrypted file:

```
bin/rails encrypted:edit config/staging_tokens.yml.enc
bin/rails encrypted:edit config/staging_tokens.yml.enc --key config/staging.key
bin/rails encrypted:show config/staging_tokens.yml.enc
```

Also provides a backing Rails.application.encrypted API for Ruby access:

```ruby
Rails.application.encrypted("config/staging_tokens.yml.enc").read
Rails.application.encrypted("config/staging_tokens.yml.enc").config
Rails.application.encrypted("config/staging_tokens.yml.enc", key: "config/staging.key")
```

Instead of stashing the values in constants.

Allow test_ignored_columns_not_included_in_SELECT column names case insensitive

insensitive

i.e. Oracle database identifier is UPPERCASE unlike other databases.

```ruby
(byebug) query = Developer.all.to_sql
"SELECT \"DEVELOPERS\".\"ID\", \"DEVELOPERS\".\"NAME\", \"DEVELOPERS\".\"SALARY\", \"DEVELOPERS\".\"FIRM_ID\", \"DEVELOPERS\".\"MENTOR_ID\", \"DEVELOPERS\".\"CREATED_AT\", \"DEVELOPERS\".\"UPDATED_AT\", \"DEVELOPERS\".\"CREATED_ON\", \"DEVELOPERS\".\"UPDATED_ON\" FROM \"DEVELOPERS\""
```

Handle `TZInfo::AmbiguousTime` errors

Fix the "International Date Line West" timezone

Make `ActiveSupport::TimeWithZone` match Ruby's handling of ambiguous
times by choosing the later period, e.g.

Ruby:
```
ENV["TZ"] = "Europe/Moscow"
Time.local(2014, 10, 26, 1, 0, 0)   # => 2014-10-26 01:00:00 +0300
```

Before:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
TZInfo::AmbiguousTime: 26/10/2014 01:00 is an ambiguous local time.
```

After:
```
>> "2014-10-26 01:00:00".in_time_zone("Moscow")
=> Sun, 26 Oct 2014 01:00:00 MSK +03:00
```

Fixes #17395.

Update configuring.md

Related to 2e0fe592

It was brought to my attention that the Rails guide suggests using filenames to ensure code load order, so I thought I would suggest a better alternative.

`oldie = 'true'` to `tinyint(1)` column is invalid value for MySQL:

```
Mysql2::Error: Incorrect integer value: 'true' for column 'oldie' at row 1: update horses set oldie = 'true'
```

Update plugin_generator.rb

Fix activesupport/CHANGELOG.md [ci skip]

Fix migration version in doc of #up_only

Performance improvements for add_method_to_attributes!

* Add a #populate method to migrations

* Address rubocop issues

* Rename to #up_only and use #execute in the examples intead of the model

* Update CHANGELOG

[Rich Daley & Rafael Mendonça França]

CI against JRuby 9.1.14.0

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

Update `exists?` documentation [ci skip]

Update layouts_and_rendering.md  [ci skip]

Make it clear that `exists?` can be chained onto a relation

Ensure plugin_generator adds to new line in Gemfile, even if the Gemfile
does not end with an empty line.

[Lisa Ugray, Nikita Penzin]

Disallow raw SQL in dangerous AR methods