Source gems from https vs. http

Is https actually more secure? o_O

Bundler 1.7 has some changes that appear to enforce
rules about multiple sources. I ran into trouble
earlier this week when testing w/ bundler 1.7.
Since this change is good regardless, it makes
sense to remove one possible compitability issue.
Merge pull request #19 from calabash/feature/Gemfile-should-source-https

Source gems from https vs. http