[C++/Python] Migrate Travis CI jobs off Xcode 6.4

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Travis CI says they are winding down their support for Xcode 6.4, which we use in our CI as the minimum Xcode which can build Arrow libraries

"Running builds with Xcode 6.4 in Travis CI is deprecated and will be removed in January 2019.
If Xcode 6.4 is critical to your builds, please contact our support team at support@travis-ci.com to discuss options.
Services are not supported on osx"

We should decide if we want to continue to support this version of Xcode, and what are the implications if we do not

Reporter: Wes McKinney / @wesm

Related issues:

• [CI] Use Ubuntu Xenial (16.04) VMs on Travis-CI (is duplicated by)

_{Note: This issue was originally created as ARROW-3255. Please see the migration documentation for further details.}

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Wes McKinney / @wesm:
What is conda-forge's plan about this?

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Wes McKinney / @wesm:
We aren't building on Xcode 6.4 anymore in CI. What is conda-forge doing?

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Uwe Korn / @xhochy:
conda-forge is using Xcode 9.4