Let Unicorn know where to find the *current* Gemfile

When using bundler and unicorn, we've had issues where unicorn would be trying to read an outdated Gemfile after a deployment from say `/var/apps/myapp/releases/c0ffee/Gemfile`, which causes the process to die until manually restarted. This patch provides the accepted fix of setting the environment's `BUNDLE_GEMFILE` variable to the version that lives under the latest symlink.