(chore) Git ignore lockfiles in gemfiles/

[olleolleolle]

This PR adds an ignore pattern to help developers not commit lockfiles for the gemfiles/ directory either.

We already had a pattern for the root Gemfile.lock.

[kaspth]

Hm. Not liking that we aren't storing the root Gemfile.lock. That's usually a bad practice that Rails doesn't follow. Also unsure about the JRuby related fix that introduced it: f99d36b

Seems like we do want these Gemfiles committed, no?

[olleolleolle]

What prompted me

• Root ignored it
• When I created a lockfile, it was with a 2.x Bundler, leaving an incompatible BUNDLED_WITH marker which made the Ruby 2.2 CI matrix elements sad

[kaspth]

• When I created a lockfile, it was with a 2.x Bundler, leaving an incompatible BUNDLED_WITH marker which made the Ruby 2.2 CI matrix elements sad

Right, but that only happened because we were ignoring the root Gemfile.lock. But either, we go all the way for one option or the other. Then there's a separate PR in committing gem lockfiles again.