Point Unicorn to current/Gemfile instead of release dir.

Without this env variable unicorn master process remembers path to
Gemfile of specific release. This causes two problems when reloading
unicorn using USR2 signal:

1. Gems are not refreshed if Gemfile has changed since the release when
unicorn was started.
2. If the release from which unicorn was started is removed due to
`keep_releases` option, unicorn will fail to reload (since that Gemfile
is no longer available).

You need to `cap stage unicorn:stop && cap stage unicorn:start`
for this fix to be applied.