Reinstate compatibility with jsonschema 2.x

[danielballan]

In #79 we made bluesky compatible with the new jsonschema 3.x API. We also chose to require that API and become incompatible with 2.x.

Unfortuantely, pip's dependency solver (unlike conda's) does not guarantee satisfying all the dependencies across steps; it just warns when it is going to ignore a pin. It often happens that our jsonschema >=3 pin gets ignored.

It would be simple enough to do a version check on jsonschema at import time and use old-style validation code for jsonschema 2.x. It adds some complexity to our codebase, but given that this issue has come up a couple times and can be puzzling if you don't know what the problem is, I think it's worth considering.