Update CI Python versions

[warsaw]

There seems to be some inconsistencies in the versions of Python tested against in the various CI integrations. From the looks of it:

• Appveyor is testing Python 2.7, 3.3, and 3.4
• Circle Is testing 2.6.8, 2.7.9, 3.4.3, and 3.5.0
• I can't tell what's being tested on Travis
• None are testing Python 3.6

I might be totally misunderstanding how these are set up, but some observations:

• Given that Python 2.6 is long EOL'd and 3.3 will be EOL'd in September 2017, it makes sense to drop both of these
• It could make sense to continue to support Python 3.4, although i would argue that many projects are dropping it due to the lack of async/await keywords. It's not unreasonable to continue to support 3.4 given that it will be supported upstream until approximately August 2018 in source-only, security-only mode.
• Only Major.Minor versions should be specified so that the highest supported patch release available is tested (e.g. 2.7 instead of 2.7.9, given that 2.7.13 is the latest upstream release and there are some potentially significant differences from .9 to .13).
• Python 3.6 should definitely be added to the test matrix.
• It might makes sense to add the current in-development version of Python (i.e. what will be 3.7 as of this writing), if the service supports it.

So as of right now, I'd say a minimal test matrix should include 2.7, 3.5, and 3.6, with 3.4 and 3.7 optionally.