add combination of Ruby 2.4 + aj:integration to CI

Modernize SQL Server database YAML for Linux/vNext.

Fix broken sample code for EventedFileUpdateChecker [ci skip]

use rails-html-sanitizer >= 1.0.3

this probably fixes #25840

CVE-2015-7579 says rails-html-sanitizer 1.0.2 has XSS vulnerability.

Bump bug report template versions to 5.0.1 in master

remove warning from `bin/setup` test

This removes the following warnings.

```
test/application/bin_setup_test.rb:43: warning: ambiguous first argument; put parentheses or a space even after `/' operator
```

https://github.com/ruby/ruby/commit/11e6bd5ac2a2eebfa589bd6db8c9c4daa337733e

Leaving the 2.4.0 conditional for now, in order never to forget backporting r57407 to 2.4.1

Action Mailer Basics clean up [ci skip]

[ci skip]

Fix grammar 'an hyphen' -> 'a hyphen' [ci skip]

This message could be generated by `assert_predicate`, https://github.com/rails/rails/commit/1853a4f2c8b2bddfbde6aae80abb08310295201c#commitcomment-20546113
but I'd rather handwrite the message string if I could reduce magic from the code by doing so.

Keep the value of `$,` and restore it

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
jruby 1.7.26 (1.9.3p551) 2016-08-26 69763b8 on Java HotSpot(TM) 64-Bit Server VM 1.8.0_45-b14 +jit [darwin-x86_64]
"\u307B\u3052"

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
jruby 9.1.7.0 (2.3.1) 2017-01-11 68056ae Java HotSpot(TM) 64-Bit Server VM 25.45-b02 on 1.8.0_45-b14 +jit [darwin-x86_64]
"\u307B\u3051\u3099"

% ruby -ve "p 'ほげ'.encode(Encoding::UTF_8_MAC)"
ruby 2.4.0p0 (2016-12-24 revision 57164) [x86_64-darwin14]
"\u307B\u3051\u3099"

Give a message to `#test_duplicable` assertion

Erubi offers the following advantages for Rails:

* Works with ruby's --enable-frozen-string-literal option
* Has 88% smaller memory footprint
* Does no freedom patching (Erubis adds a method to Kernel)
* Has simpler internals (1 file, <150 lines of code)
* Has an open development model (Erubis doesn't have a
  public source control repository or bug tracker)
* Is not dead (Erubis hasn't been updated since 2011)

Erubi is a simplified fork of Erubis that contains just the
parts that are generally needed (which includes the parts
that Rails uses).  The only intentional difference in
behavior is that it does not include support for <%=== tags
for debug output.  That could be added to the ActionView ERB
handler if it is desired.

The Erubis template handler remains in a deprecated state
so that code that accesses it directly does not break.  It
can be removed after Rails 5.1.

Giving a message helps us to know what happened
when we look at Travis CI.

remove unreachable code

As unit tests, we do not know the value of `$,` when this
test case started. It' better to keep the value when the
test case fnished.

Fix sample code for `expand_cache_key` usage [ci skip]

Delete PATH_INFO after each controller test request

Prevents PATH_INFO from being used to infer the request format in later
test requests when no explicit format is given.

As the request PATH_INFO may be set before a request, it can't be
deleted during pre-request scrubbing.

Fixes #27774

do not run `git init` when specify the `pretend` option

`app_path` is a required argument. Therefore, if `app_path` is not
specified, it becomes an error in thor's option parsing, does not reach
this check.

When specifying the `pretend` option, expect that no processing will be
done, so do not execute `git init` as well.

Fixes "NameError: uninitialized constant ApplicationTests::ConfigurationTests::CustomTest::Rails"
when run individually

Add module name to BacktraceCleaner usage example [ci skip]

Make BigDecimal casting consistent on different platforms

Right now it behaves differently on JRuby:

```
--- expected
+++ actual
@@ -1 +1 @@
-#<BigDecimal:5f3c866c,'0.333333333333333333',18(20)>
+#<BigDecimal:16e0afab,'0.3333333333333333',16(20)>
```

My initial PR (https://github.com/rails/rails/pull/27324)
offered to let the precision to be decided by the platform and
change the test expection, but other contributors suggested
that we should change the default precision in Rails
to be consistent of all platforms.

The value (18) comes from the max default precision that comes
from casting Rational(1/3) to BigDecimal.