Fix gem deps so tests run under Ruby 1.9.2 and REE

* ActiveModel 4 requires Ruby >= 1.9.3, so ActiveModel 3 must be
  installed instead.
* Guard depends on the listen gem. Listen 2 also requires >= 1.9.3,
  so listen 1 must be installed instead.
Merge pull request #112 from mcmire/emw-fix-running-tests-on-1.9.2

Fix gem deps so tests run under Ruby 1.9.2 and REE