revert back to ruby 2.1

- - -

Revert Ruby 2.2 changes - until we solve https://gitlab.com/gitlab-org/gitlab-ce/issues/13514

This also updates Ruby 2.1.7 to 2.1.8

See merge request !2898

Add ability to revert changes introduced by Merge Requests or Commits

Closes #3409 

See merge request !1990

Consistency in MR and issue header.

Fixes: #13550 

![Screen_Shot_2016-02-18_at_11.20.19_AM](/uploads/8327ff045e45029b85b2cbac72c98a7f/Screen_Shot_2016-02-18_at_11.20.19_AM.png)![Screen_Shot_2016-02-18_at_11.20.55_AM](/uploads/f79613ebb23e4a62fb1bdb48d6f7fd2b/Screen_Shot_2016-02-18_at_11.20.55_AM.png)

See merge request !2874

I forgot to update this and some spinach specs were failing.

[ci skip]

* Now it's only created when user choose to create a MR and
  logic to hide it is handled by https://gitlab.com/gitlab-org/gitlab-ce/blob/50595af7b0e304f16e2955109abd2d25cd96efda/app/helpers/application_helper.rb#L139

* Now checking if a commit is already reverted or there is a conflict is
  much more faster.
* No longer need to create a new branch.