Understand (and fix) `increment` problem

`Rails.cache` was returning differently typed objects, due to a problem
in the `dalli` gem.  This has been fixed by @sj26 in (and requires)
envato/marketplace#6549

Now that that problem is gone, we no longer need any special handling in
this class, and it can return to being (wonderfully) simple:

- Removed `failure_count=`, since it's never called (and would require
  the use of `raw` mode)
- Go back to using the standard `increment` method
- Change `reset_failure_count` back to deleting the object, rather than
  writing `0` to it