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ARROW-628: [Python] Install nomkl metapackage when building parquet-cpp in Travis CI

I was surprised to find conda installing the mkl conda package with what's there now, but this should fix that and make the builds slightly faster

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

Closes #380 from wesm/ARROW-628 and squashes the following commits:

3598dcf [Wes McKinney] Install nomkl metapackage when building parquet-cpp in Travis CI