Reduce gem dependencies to railties

[csutter]

Describe the change

Currently the gem depends on the entirety of rails, which adds some
"dependency weight" to projects that only need a subset of Rails
functionality. This changes the gem to depend on railties instead,
which covers all of the core Rails functionality Loaf needs to function.

Why are we doing this?

Fixes issue #44

Benefits

Installing Loaf will no longer lead to additional Rails sub-gems being installed beyond what Loaf needs to function (e.g. actioncable, active storage etc.) beyond what the host application requires.

Drawbacks

• A very edge-y edge case could be apps that rely on some parts of Rails that Loaf previously added an implicit dependency for
• It's difficult to do automated testing for this change - I've tested my application that uses Loaf locally against this change, and it worked fine, as did the test Rails app but that obviously requires all of Rails (and I don't necessarily want to change that given that it's what most users are likely to be doing)

Requirements

Put an X between brackets on each line if you have done the item:
[x] Tests written & passing locally?
[x] Code style checked?
[x] Rebased with master branch?
[x] Documentaion updated?

[coveralls]

Coverage remained the same at 96.954% when pulling 5bf390e on csutter:rails-to-railties into 6a8d586 on piotrmurach:master.

[piotrmurach]

Perfect! Thank you ❤️

[piotrmurach]

Released v0.10.0 with your contribution.