Add code coverage via coveralls.io

https://coveralls.io/r/twitter/secureheaders

Signed-off-by: Chris Aniszczyk <zx@twitter.com>
Committing and pushing because I can't install 1.8.7 for some reason...
Merge pull request #120 from twitter/add-coveralls

Add code coverage via coveralls.io
Merge branch 'master' into script_hashes_again

Conflicts:
	Gemfile