Add an --ignore-branch option for 'gemnasium push'

Mostly copied from https://github.com/gemnasium/gemnasium-gem/pull/4, but renamed and including the additional requested spec. I've also tested this locally w/ a project and ensured that Gemfiles were pushed to Gemnasium regardless of branch when the ignore-branch flag is used.

The motivation for this is that we run `gemnasium push` on our Jenkins server and it's running the tests in a detached branch. For example, one of our jobs' `current_branch` value is "(detached from 8e1a2b8)"). This is preventing us from keeping our gems up to date on Gemnasium.

Please let me know if there's anything else needed for a merge.