Only invoke route.source_location if available

[vinistock]

Closes #492

Fix for #488

Since the Rails add-on is installed by default, while we don't aim to have every feature work on EOL versions of Rails, we still need to ensure we don't break. The source_location method only exists after 7.0.

It would be nice to generalize a solution for handling which features are available based on Rails version. Rather than checking with respond_to? on the server side every time, we can avoid the request to the server completely if we keep some sort of feature dictionary. Something like

SERVER_FEATURE_VERSIONS = {
  route_locations: "7.1",
  some_other_thing: "8.0",
}