[Python] Add Python 3.6 to Travis CI

[asfimport]

We need to make sure the next release of PyArrow works on Python 3.6

Reporter: Wes McKinney / @wesm
Assignee: Wes McKinney / @wesm

Related issues:

• Arrow 0.3 release (blocks)

_{Note: This issue was originally created as ARROW-741. Please see the migration documentation for further details.}

[asfimport]

Wes McKinney / @wesm:
PR: #514

[asfimport]

Wes McKinney / @wesm:
Issue resolved by pull request 514
#514