Lookup policies in the current namespace

Addresses #12. If the policy is not defined in `namespace`, `const_get`
will search through the inheritance change of `namespace` to find the
policy.
Use `rbx-2` in .travis.yml due to regression in rvm

See:
https://twitter.com/travisci/status/467072346231025665
Merge pull request #153 from ecbypi/fix-travis-yml

Use `rbx-2` in .travis.yml due to regression in rvm
Merge pull request #152 from ecbypi/12-namespaced-policies

Lookup policies in the current namespace
Change @policy instance variable to @_policy.
Merge branch 'DylanFM-policy_instance_variable'