remove rvm global gems override

[radilr1]

1. The override of rvm global gems in the attributes overrides the settings in the rvm recipe too. This makes it impossible for the user to define any other global gems except these in the cookbook.
2. I changed the rvm recipe a little bit to accept object not just string in case the user wants to change the version this fixes another problem:
   With the latest rails version 4.0.3 the default bundle version 1.5.0 cant install some of the gems (minitest 4.7.5).