Access all git dependencies using https

git:// URIs can be problematic as the port used by the git protocol
(9418) is often blocked for outbound connections by a firewall. The
Gemfile was using a mixture of git:// and https:// URIs for its
Git-based dependencies. This commit makes them all use https.
Merge pull request #26 from wilkinsona/git-dependencies

Access all git dependencies using https