Specification of ActiveModel Version During Testing

The following adds the ability to test the validator against various versions of ActiveModel:

```sh
rake test # defaults to '~> 3'

# Specify a rails version with the pessimistic version operator
ACTIVE_MODEL=4.0.0 rake test # '~> 4.0.0'

 # Run tests against master branch on GitHub
ACTIVE_MODEL=master rake test
```

Travis configs have been updated to test for multiple Rails versions as well