If Git is not available, prints better error message when using gems from git

https://github.com/bundler/bundler-features/issues/11

error message if git is not installed

add method to store git version if found during bundle install or update

update code to check from the  method in bundler.rb

move spec test for checking if git is installed

remove old spec file

clear the path so Bundler can't find git

remove whitespace

remove whitespace galore and change error message to be more useful

update rspec test to match more useful error message in git_proxy.rb