Refactor Python to use `update` mode.

- Also fix rpath issue on OS X
- Relies on https://github.com/hashdist/hashdist/pull/294
Accommodate Python version more gracefully

I've switched common.yaml to config.yaml to make it more clear
that this is a file you may want to modify.

Perhaps common_config.yaml would be even better?
Merge remote-tracking branch 'origin/master' into ahmadia/refactor_python_update
Added 3.3 and 3.4 tests
Merge pull request #559 from hashdist/ahmadia/refactor_python_update

Refactor Python to use `update` mode.