Fixed build errors due to recent changes (homebrew/pip)#376

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starrify:fix-build

Commits on Oct 24, 2019

Fixed build error due to recent homebrew update
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Specifying "2.7.9" (relatively newer) instead of "2.7" in travis config since pyenv may seriously install Python 2.7.0
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Commits on Oct 25, 2019

Requesting pip<19.3 for now due to incompatibility (pip._internal.main no longer callable)
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Commits on Nov 18, 2019

Removed the pip<19.3 constraint
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