Add database cleaner support file

This adds a database_cleaner.rb file to spec/support to setup database_cleaner for rspec. See @jferris's comment [here](https://github.com/thoughtbot/suspenders/commit/d473f459c3727211bb77f2e25611a1ebc9d5edb1#L0L24).

Also, the rake_clean.feature was having false positives. The rake tasks were ran in the wrong directory, and we didn't check rake task's exit status. This PR fixes them.

Details:
* Add database_cleaner spec/support file to setup db cleaning for tests
* Drop databases prior to suspending an app (clearance acts different
*   based on whether users table exists or not)
* Verify that each rake tast succeeds
* Use clear rake run step