Drop Python 3.3 support

[richardotis]

It looks like building "clean" conda packages supporting Python 3.3 isn't possible anymore without leaning on pip, which is what we do on Travis CI. We can still test against it, but it's probably best to say 3.4 is the minimum supported version now.

[richardotis]

On further thought, I think it is best to drop explicit Python 3.3 support altogether. There is no point in testing against a platform we can't support if something breaks.