Load New Gemfile on Rolling Restart

When `bluepill start` is run it uses the release folder Gemfile. This means that when `bluepill restart` is run it is still pointing to the old Gemfile; if extensions were added/updated the restarted unicorn process won't pick it up.

This change forces unicorn to look at the most recent release's Gemfile.

**References:**

* http://unicorn.bogomips.org/Sandbox.html
* http://davidvollbracht.com/blog/headachenewunicorn-capistrano-bundler-usr2