Dwolla integration: Fix HMAC dependency under 1.8

Ruby 1.8 does not have digest/hmac, so it's necessary to use OpenSSL
until we drop 1.8 support.
Balanced: Mock out HTTP request in unit test

One of the refund tests wasn't mocking out the request that the gateway
was making, and since it was expecting failure it was still passing.
Found this by being on a plane without connectivity.