Made python 3 support official

[richardjgowers]

Fixes #260

Changes made in this Pull Request:

• py 3.6 build can no longer fail
• added python 3.4 and 3.5 to build matrix

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[richardjgowers]

I need to go through and check where we state that we don't support py3

[tylerjereddy]

For the build matrix failures, I think we've been starting to see these a bit more (randomly?) now:

==================================== ERRORS ====================================
_____________________________ ERROR collecting gw1 _____________________________
Different tests were collected between gw0 and gw1. The difference is:

There's an associated discussion on the pytest github, which then broke out to an xdist issue.

Is this serious enough to warrant removing the parallel testing for now? We need reproducible CI test results and cycles are currently slow / expensive / etc. Maybe a new issue needs to be opened for the discussion.

[richardjgowers]

Ok I disabled xdist for now, build times are about 30mins maximum, which is manageable for now. Should be good to merge

[tylerjereddy]

LGTM. For the separate issue of the ignored exception in the 3.4/3.5 rows of the build matrix, see #1530.

[tylerjereddy]

Perhaps I'll allow another @MDAnalysis/coredevs to merge, given the impact of changing the Travis build matrix & need for agreement, etc.

[orbeckst]

@richardjgowers because this PR changes the CHANGELOG with the momentous entry "we support Py 3 now", I would also like the same PR remove any of the "Py 3 is experimental" warnings as well as updates to the docs that clearly state what we're supporting; In #1446 we got the justified complaint that our docs are not clear on what we support.

[richardjgowers]

@orbeckst that notice is already killed

Because the request changes were done elsewhere.