Replace AS::TimeWithZone#since with alias to +

Stems from [Google group discussion](https://groups.google.com/forum/#!topic/rubyonrails-core/jSPbP-TNLb0).

Currently `AS::TimeWithZone` has two methods to add an interval to a time:
`+(other)` and `since(other)` ([docs](http://edgeapi.rubyonrails.org/classes/ActiveSupport/TimeWithZone.html)).

The two methods are "pretty much" equivalent in every case:

1. When adding any interval to an `AS::TimeWithZone` representing a `Time`:

  ```ruby
  t = Time.now.in_time_zone       #=> Thu, 04 Dec 2014 18:56:28 EST -05:00
  t + 1 == t.since(1)             #=> true
  t + 1.day == t.since(1.day)     #=> true
  t + 1.month == t.since(1.month) #=> true
  ```

2. When adding any interval to an `AS::TimeWithZone` representing a `Date`:

  ```ruby
  d = Date.today.in_time_zone     #=> Thu, 04 Dec 2014 00:00:00 EST -05:00
  d + 1 == d.since(1)             #=> true
  d + 1.day == d.since(1.day)     #=> true
  d + 1.month == d.since(1.month) #=> true
  ```

3. When adding any interval to an `AS::TimeWithZone` representing a `DateTime`:

  ```ruby
  dt = DateTime.now.in_time_zone    #=> Thu, 04 Dec 2014 18:57:28 EST -05:00
  dt + 1 == dt.since(1)             #=> true
  dt + 1.day == dt.since(1.day)     #=> true
  dt + 1.month == dt.since(1.month) #=> false
  ```

As you can see, the only case in which they differ is when the interval added
to a `DateTime` is in a format like `1.month`.

However, this usage of "since" is explicitly discouraged by the
[documentation of `DateTime#since`](https://github.com/rails/rails/blob/master/activesupport/lib/active_support/core_ext/date_time/calculations.rb#L86L88):

> Returns a new DateTime representing the time a number of seconds since the instance time.
> Do not use this method in combination with x.months, use months_since instead!

And indeed, following this recommendation the correct result is returned:

```ruby
dt + 1.month == dt.months_since 1 #=> true
```

Therefore, my proposal is to remove the method definition of `TimeWithZone#since`
and instead replace it with a simple `alias_method :since, :+`.

The rationale is that the only case where they differ is a case that is
explicitly discouraged as "wrong".

In my opinion, having two methods named `since` and `+` and having to figure
out exactly what the difference is makes the codebase more confusing.

However, I understand this PR is "subjective", so if you feel like it's better
to ignore this, feel free to close the PR.

Thanks!
Remove unused line
[ci skip] Change three backticks to just one
Merge pull request #18069 from yui-knk/fix/guide

[ci skip] Change three backticks to just one
Merge pull request #18068 from kamipo/remove_unused_line

Remove unused line
use thread_safe gem version greater or equal to 0.3.4
simpler example for the non-missing constants gotcha [ci skip]

The previous example was a little convoluted and the exposition
claifying the parts that were correct albeit not totally obvious
were interferring in my view.

This example has less things going on and gets to the key problem
with less balls in the air.
Merge pull request #18072 from exAspArk/thread_safe_gem_version

Use thread_safe gem version greater or equal to 0.3.4
Merge pull request #18063 from claudiob/remove-as-time-with-zone-since

Replace AS::TimeWithZone#since with alias to +
Fix name on changelog [ci skip]
Merge pull request #18076 from andreynering/guides-changelog

Fix name on changelog [ci skip]
Fix typo in nodoc should be `:nodoc:` for RDoc to parse correctly
Allow precision option for PostgreSQL and MySQL datetimes and times.
Format the datetime string according to the precision of the datetime field.

Incompatible to rounding behavior between MySQL 5.6 and earlier.

In 5.5, when you insert `2014-08-17 12:30:00.999999` the fractional part
is ignored. In 5.6, it's rounded to `2014-08-17 12:30:01`:

http://bugs.mysql.com/bug.php?id=68760