.gitignore Gemfile.lock per previous commit

Gemfile.lock was removed in a previous commit, should be ignored so it doesn't get committed again.

https://github.com/imathis/octopress/commit/06c04397053e46fe02fcecce589a2de76d21e160
Merge pull request #1617 from nickhammond/patch-1

.gitignore Gemfile.lock per previous commit
Delete Gemfile.lock

Follow-up to #1617