Closes gitaly#355

Since asset compilation was disabled in GitLab 9.3, the premailer-rails gem
would fall back to fetching assets via HTTP(s) since the Sidekiq working
directory was not the Rails root. In some instances, SSL verification would
fail, preventing e-mails from being sent.

premailer-rails 1.9.7 has a fix
(https://github.com/fphilipe/premailer-rails/pull/195) that uses an absolute
directory to load assets from the filesystem so that the assets can be fetched
locally instead of over the network.

Closes #34231

This reverts commit 95138a9c, reversing
changes made to 66aad16d.

Changelog: https://github.com/carrierwaveuploader/carrierwave/blob/f2f8569b1013f601fe7be4b3a616359acd27240d/CHANGELOG.md#110---2017-04-30

Adds support for Rails 5.1

Changelog: https://github.com/sickill/rainbow/blob/master/Changelog.md#222-2017-04-21

Adds support for Ruby 2.4.

https://github.com/rubysherpas/paranoia/blob/core/CHANGELOG.md#230-2017-04-14

Adds support for Rails 5.1.

Closes gitlab-org/omnibus-gitlab#1714

Signed-off-by: NRémy Coutable <remy@rymai.me>

Signed-off-by: NRémy Coutable <remy@rymai.me>

+ Use NullMetrics to mock metrics when unused
+ Use method_missing in NullMetrics mocking
+ Update prometheus gem to version that correctly uses transitive dependencies
+ Ensure correct folders are used in Multiprocess prometheus client tests.
+ rename Sessions controller's metric

+ Cleanup Gemfile

Closes gitaly#199

It was only available for test and development environments and we need
it to run the `rake gettext:pack` command in omnibus which doesn't have
the test/dev gems installed.

This gem prevents Mash from responding to :permitted?, disabling mass
assignment protection for the Grape API

Closes gitaly#173