Retrieve Active Admin gem over SSL/TLS;

So, unfortunately, the `github:` shorthand in Gemfiles resolve to the `git://` protocol which is, surprisingly, insecure.

The rubysec organization recommends that all Gem sources should be accessed over SSL/TLS.
Merge pull request #84 from f3ndot/use-secure-gem-sources

Retrieve Active Admin gem over SSL/TLS;