Adding Python3.7 wheels support

[devin-petersohn]

What do these changes do?

Build Python 3.7 wheels during travis tests.

Related issue number

#1115

[robertnishihara]

Nice!

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[robertnishihara]

Whether this works or not just depends on whether the base image already has Python 3.7 on it. The base image is the one used for building Arrow wheels.

[pcmoritz]

we should revisit this, arrow now has support for 3.7 I think; see also #1115

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[pcmoritz]

Note: I made sure this works with the earliest numpy compatible with python 3.7 on mac, see changes in the PR.