Depend only on railties instead of rails

[langalex]

By making rails a dependency, any app adding this gem will inherit all the Rails gems such as activerecord, actioncable, actionmailer, activestorage etc., even if it is not using these optional dependencies itself.

By changing the dependency to just railties, no optional Rails dependencies are forced upon apps.

fix igorkasyanchuk/rails_performance#60

[langalex]

Could this be merged soon? It's a trivial PR that is not going to affect anything but the dependency tree of apps. Thanks.

[igorkasyanchuk]

merged 👍 thanks