Fixes to gemfile

* Now specifying Ruby 2.0.0
* Updated 'postgres'to 'pg'

Discovered some outdated dependencies on my machine so I
fixed these along with the Gemfile references. Upon upgrading,
bundler informed me that I was using an outdated reference to the
Postgres driver and needed to use 'pg' instead of 'postgres' in
the Gemfile.
reverted to ruby 1.9.3 due to pg gem incompatibility on Windows
Finished tests on Invite model, removed Factories

I had some difficulty combining FactoryGirl with instantiated
objects, so I removed all references to factories. Not really
needed for a project of this complexity right now.

Finished the 2 tests & respective validations:
* does not allow you to use Invite when status is not active
* does not allow you to use Invite when associated user has
  a status that is not active