Fix Travis

This PR includes several changes:

1. Added Ruby 2.0 and 2.1.0 to the Travis matrix
2. Updated the default Gemfile to use ActiveModel ~> 3.2 as appears to have been intended.  That ensure it has a different gemset than Gemfile_activemodel4
3. Fixed a typo in the .travis.yml - 'ruby' should be 'rvm'
4. Updated the lockfiles to use recent gems - this eliminated a number of warnings and allowed me to run specs locally.
5. Fixed an rspec deprecation warning

With this PR everything runs green on Travis.  You can see a build here - https://travis-ci.org/petergoldstein/Dynamoid/builds/16948616

A few quick comments:

1. While the .travis.yml is in the repo, it's not clear to me that Travis builds were ever activated for the core repo.  Looking here - https://travis-ci.org/Veraticus/Dynamoid - I don't see any builds, now or in the past.  A committer may have to activate builds at http://travis-ci.org
2. For reasons that are unclear, running rspec on Travis is taking much, much longer than locally. I'm seeing about 30 seconds locally, and about 12 minutes on Travis.  Not sure what the difference is.