Upgrade to mongoid ~> 3.0.
Merge pull request #6 from simi/mongoid-3.1

Upgrade to mongoid ~> 3.0.
Start mongoid for Travis tests.
Merge pull request #7 from simi/travis-fix

Start mongoid for Travis tests.
Minor update to CHANGELOG, one line.
Incremented version to 0.6.1.
Incremented version to 0.6.2.
Regenerated gemspec with 0.6.2.
Updated README with Mongoid compat versions.