Fetch activeadmin from Rubygems instead of git

[sauloperez]

When running bundler commands in the repo a warning shows up:

$ ➜  timeoverflow (upgrade-to-rails4.2.7.1) bundle update rspec-rails                                                                       The git source `git://github.com/activeadmin/activeadmin.git` uses the `git` protocol, which transmits data without encryption. Disable this
 warning with `bundle config git.allow_insecure true`, or switch to the `https` protocol to keep your data secure.

We should fetch that gem from Rubygems instead of git for security reasons.

[markets]

Closed by #298