Add the git url in the README.md install instructions.

After following the installation instructions I got the following error:

$ bundle
Fetching gem metadata from https://rubygems.org/.........
Fetching gem metadata from https://rubygems.org/..
Resolving dependencies...
Could not find gem 'guard-nosetests (>= 0) ruby' in the gems available on this machine.

I manage to make it work by adding the url of the git repo to the Gemfile. This might help the next users while the issue with rubygems.org is fixed.