Depend on `railties` and not `rails`.

The `rails` gem is a meta gem to glue all the other ones together
and shouldn't be used as a runtime dependency of other gems.
This way Peek won't bring `activerecord` or `arel` as dependency
on applications that aren't using Active Record.
Merge pull request #57 from lucasmazza/lm-rails-dependency

Depend on `railties` and not `rails`.