Bump paper_trail gem.

Version 3.0.3 has been yanked, and is failing to install if not already
present.
Merge pull request #100 from alphagov/fix_paper_trail

Bump paper_trail gem.
Remove unicorn config overrides.

This means that it will use the same defaults as most of the other apps.
Specifically, this will reduce the number of workers to 2 per box in
line with the other publishing app.  This should relieve some of the
memory pressure on the backend servers.