Link for "New Post" is added in Getting Started guide [ci skip]

- Earlier(line number 862) it is said to add <%= link_to 'New post', new_post_path %> on posts index page before table tag. But when posts index page html is shown, "New Post" link is missing.
So aaded "New Post" link in this commit
Dropped yajl from Gemfile
Requires JSON gem version 1.7.7 or above as it contains an important
security fix.
Deprecated Numeric#{ago,until,since,from_now}

The user is expected to explicitly convert the value into an
AS::Duration, i.e. `5.ago` => `5.seconds.ago`

This will help to catch subtle bugs like:

  def recent?(days = 3)
    self.created_at >= days.ago
  end

The above code would check if the model is created within the last 3
**seconds**.

In the future, `Numeric#{ago,until,since,from_now}` should be removed
completely, or throw some sort of errors to indicate there are no
implicit conversion from `Numeric` to `AS::Duration`.

Also fixed & refactor the test cases for Numeric#{ago,since} and
AS::Duration#{ago,since}. The original test case had the assertion
flipped and the purpose of the test wasn't very clear.
Merge pull request #13069 from chancancode/json_gem_version

Requires JSON gem version 1.7.7 or above as it contains an important security fix.
Merge pull request #13068 from chancancode/drop_yajl_from_gemfile

Dropped yajl from Gemfile
Merge pull request #13067 from sagarjunnarkar/getting_started

Link for "New Post" is added in Getting Started guide [ci skip]
Merge pull request #13072 from kuldeepaggarwal/unwanted-code

unnecessary checking of `size` with `second regex` if matched with former regex
Merge branch 'deprecate_numeric_since_ago' of https://github.com/chancancode/rails

Conflicts:
	activesupport/CHANGELOG.md