Refactor vernier support logic

[gmcgibbon]

We use a defined? check to make sure the vernier constant can be loaded. Instead, we should be able to determine which config option maps to what backend without loading said backend. This was recently re-added to App Profiler in #154 after the behaviour regressed. So, I'd like to:

1. Change the vernier branching logic not use defined? by keeping backend names outside of their respective constants that load peer dependency gems.
2. Make stackrof a peer dependency as well (if it isn't already).
3. Add tests to make sure we don't regress.