Fix gemfile

1. replace `"` to `'` in Gemfile.

2. change `require: nil` to `require: false`.
`require: nil` and `require: false` get same result, but i think we shouldn't write it differently in other places.

3. set version for gems.
better gem's version control for prevent dependency errors.

See merge request !5078

Upgrade oauth2 from 1.0.0 to 1.2.0.

Changelog: https://github.com/intridea/oauth2/compare/v1.0.0...v1.2.0

Follow-up on !3434 since 1.2.0 doesn't limit the JWT version we can use.

Adds support for Rails 5. Working toward #14286.

See merge request !5041

Add lib/gitlab/emoji.rb instead of gitlab_emoji gem and upgrade Gemojione

- No reason to split it into a separate gem when the gem barely did anything. We can use gemojione directly, making updating gemojione that much easier. Unless there's a particularly good reason we were using the gem?
- Fixes the Rake task since it broke after all the AwardEmoji changes.
- Update gemojione to 2.6.1.

Spring Update changes! http://emojione.com/releases/#2.2.0

See merge request !4919

Upgrade Doorkeeper from 3.1.0 to 4.0.0.

Includes Rails 5 support and various bug fixes. Working toward #14286.

Changelog: https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#400

See merge request !5055

better gem's version control for prevent dependency errors.

require: nil and require: false get same result, but i think we shouldn't write it differently in other places.

Upgrade rspec-rails from 3.4.2 to 3.5.0.

Also upgrade its dependencies. Includes Rails 5 support. Working toward #14286 

Changelogs: http://rspec.info/blog/2016/07/rspec-3-5-has-been-released/

See merge request !5051

Includes Rails 5 support and various bug fixes.

Changelog: https://github.com/doorkeeper-gem/doorkeeper/blob/master/NEWS.md#400