Make sprockets-rails an optional dependency

[rzane]

Summary

This gem should not require downstream applications to use sprockets. This gem does provide a JavaScript asset under app/assets/javascript and it does enhance the assets:precompile with additional functionality, but these things don't necessarily depend on sprockets-rails, so it sprockets-rails is really more of an optional dependency.

The dummy app does currently depend on sprockets-rails, so this should be development dependency.

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/domain @Betterment/test_track_core
/platform @Betterment/test_track_core

[samandmoore]

domainlgtm platformlgtm

it's a bit of a funky change to release since it's ultimately not an explicit dependency today either, but i think it's a wise change to better communicate the intention.