Enable refremote specs

Enables the `refRemote` tests from the common test suite by using webmock to stub out requests to `http://localhost:1234/<foo>`. After this, there's only one mandatory test remaining (draft3 disallow) for compliance with the common suite (see #163).

Two annoyances here, but I think both are minor enough to be acceptable for now:

1. The webmock usage currently bleeds into another test (test_bad_schema_ref), where we need to explicitly re-enable network access in order for the expected error to be raised. It's just the one file (so far?), so I didn't spend much time trying to address this. webmock seems to stub out Net::HTTP as soon as it is required.
2. A test run is somewhat slower now: from ~1.5s to ~2.5s on my machine. Probably an acceptable tradeoff, but a bit disappointing. =)