1411 add rspec

In this PR I added rspec. There weren't any tests in the project yet, so I just added rspec out of habit. I'm open to other suggestions, but it's rspec for now.  Once rspec was added, I wrote some specs for the `StreetUsePermit` class (which powers the endpoint with the same name). Also with testing in place, I made some minor refactors & fixes to the `HashCache` and `StreetUsePermits` classes.