Remove Gemfile assets group

Recommendation in the following article is to remove this group when
using backported asset pipeline-related gems:
https://discussion.heroku.com/t/using-the-rails-4-asset-pipeline-in-rails-3-apps-for-faster-deploys/205

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Fix require of renamed OpenProject::Hooks module

See ee70ffbc.

Signed-off-by: Alex Coles <alex@alexbcoles.com>
Attempt workaround for failing theme asset specs

Signed-off-by: Alex Coles <alex@alexbcoles.com>