Revert "Ignore Gemfile.lock"

It turns out that a Gemfile.lock is indeed needed for Heroku deployment, so here is a revert commit to put it back in place.

This reverts commit 800352a32c1b6c0c6fee08b1e33d70300a2b0414.

Signed-off-by: Emanuele <feliziani.emanuele@gmail.com>