take VCR cassettes out of repo

In #2036 we sprouted the ability to record HTTP sessions during testing and then reuse the result of those sessions in subsequent tests. This is done via the VCR library (a port from Ruby). Storing the cache in the repo doesn't seem quite right to me, so in this PR I'm proposing we take it out. The implication is that the first run of the test suite will take a loooong time to prime the cache. Subsequent runs should be an order of magnitude faster (back down near where we were before).