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ARROW-741: [Python] Switch Travis CI to use Python 3.6 instead of 3.5

I'm OK with not building Python 3.5 in Travis CI anymore because 3.5 and 3.6 are essentially the same at the C API level. Other opinions?

Author: Wes McKinney <wes.mckinney@twosigma.com>

Closes #514 from wesm/ARROW-741 and squashes the following commits:

3aee721 [Wes McKinney] Remove apache channel
116b229 [Wes McKinney] Switch Travis CI to use Python 3.6 instead of 3.5