Always define reader for keys

This only worked previously if the custom methods were defined before
the call to `map`. Since all custom methods are defined on the
`Paraphrase::Params` class, the `Query` class will never respond to the
method at the time `map` is called.
Update gemfiles, test on Rails 4.1
Remove `_param_processor` class attribute

Avoid thread-safety issues in lazily defining properties on the class.