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.
Merge pull request #1 from OlivierBlanvillain/patch-1

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