Updated documentation for 7.5 release

Documentation for 7.5 release has been updated. Changes include:

- Pointed Ruby version in guides to 2.1.5
- Pointed gitlab-shell version in guides to 2.2.0
- Links to configuration samples for EE files point to the Subscribers' EE repo

See merge request !244

Renamed main update file to ...7.5.md, updated links and versions (Ruby to 2.1.5 and gitlab-shell to 2.2.0).

 Corrected ruby download link.

Improved git hook description

https://dev.gitlab.org/gitlab/gitlabhq/issues/1741

See merge request !243

Update changelog

See merge request !1258

Run 'GC.start' after every EmailsOnPushWorker job

This change removed a lot of large peaks of the Sidekiq memory growth graph on gitlab.com.

See merge request !1257

Update gitlab_git to 7.0.0.rc11

See merge request !1256

Gitlab.com uses special packages, build them first

See merge request !1255

Author attributes clean

See merge request !1254

Strip tags from branch name

See merge request !1251

Update changelog

See merge request !1252

Update changelog

See merge request !240

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

Update EE changelog

See merge request !239

CE master to EE master

See merge request !238

Conflicts:
	Gemfile
	VERSION
	app/assets/javascripts/project.js.coffee
	app/assets/javascripts/users_select.js.coffee
	app/controllers/projects/services_controller.rb
	app/models/project.rb
	app/views/groups/edit.html.haml
	features/project/service.feature
	features/steps/project/services.rb
	spec/models/concerns/mentionable_spec.rb

Fix for #8271: Correctly restore empty repositories.