Poor man's debugging

`failure_count` is returning several different types:
- `nil` (when called with an empty cache)
- a `Fixnum`
- an `ActiveSupport::Cache::Entry` object
Revert "Poor man's debugging"

This reverts commit 8b84cd4052dd19ca53db424f1f5dc557ea73d9fd.
Fix issues with different types from cache

`Rails.cache` was returning differently typed objects, due to a problem
in the `dalli` gem.  This has been fixed by @sj26 in (and requires)
https://github.com/envato/marketplace/pull/6549

Now that that problem is gone, we no longer need any special handling in
this class.

- 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
Merge pull request #2 from envato/understand-increment-problem

Understand (and fix) `increment` problem