Add Gemfile.lock to .gitignore

[kanutocd]

Summary

Ignore Gemfile.lock in the repository.

bundle install generates Gemfile.lock, but the repository does not track it. Ignoring the generated lockfile keeps the working tree clean after the standard development setup.

Before

$ bundle install
$ git status

Untracked files:
  Gemfile.lock

After

$ bundle install
$ git status

nothing to commit, working tree clean