Remove gem files from gitignore list

There's no need to ignore these, and not seeing them when running `git
status` can sometimes confuse you when you've just built a gem.

Change-Id: I21fc4545f0913866e8a125b69d0cd3843db720e8
Reviewed-on: http://gerrit.causes.com/45138
Reviewed-by: Shane da Silva <shane.dasilva@brigade.com>
Tested-by: Shane da Silva <shane.dasilva@brigade.com>