replace rails dependency with railties

[brycekwan]

This removes the rails dependency from the poirot and replaces it with railties. This was tested using the example application.

The rails dependency pulls in unnecessary gems such as activerecord, which is not needed if you are using another ORM.

[olivernn]

I think you would also need to add ActionPack as a dependency for the template helper, see here https://github.com/olivernn/poirot/blob/master/lib/poirot.rb#L7-8

[brycekwan]

Railties already pulls in ActionPack as a runtime dependency. So we don't need to specify it as a dependency.