Improve security for the rubygems on GitHub with https instead of git

[kamataryo]

Hi, Tim,

When installing the rubygems on GitHub, three of them raise warnings.

This looks to be a feature of bundler >=1.13.0. https://github.com/bundler/bundler/blob/1-13-stable/lib/bundler/dsl.rb#L268..L273

This PR declares usage of https, remove the warnings and improve users security.

vagrant@vagrant-ubuntu-trusty-64:/srv/mapwarper$ bundle install
The git source `git://github.com/timwaters/audited.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.
The git source `git://github.com/timwaters/actionpack-action_caching.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.
The git source `git://github.com/rails-api/active_model_serializers.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.
Fetching gem metadata from https://rubygems.org/.........