Freeze mypy version for CI (fixes #408)

[cxong]

This freezes the version of mypy in the pip install command in .travis.yml and .appveyor.yml to the one in Pipfile.lock (0.600).

A better solution would be to get CI to use pipenv to install its dependencies, but I ran into issues setting that up for Travis CI; I'm not sure if pipenv currently works well in Travis CI. Haven't tried this for AppVeyor yet.

[coveralls]

Pull Request Test Coverage Report for Build 587

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.004%

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Totals
Change from base Build 585:	0.0%
Covered Lines:	2619
Relevant Lines:	2728

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💛 - Coveralls

[zsol]

Thanks @cxong! Yep, this will make the builds pass, but it doesn't actually fix #408. I think we'll need to do both of

1. use the pinned package versions for CI
2. fix the actual type errors and bump the mypy version

What were the issues you ran into with pipenv + travis?

[zsol]

I did 2. from above, so I'm going to close this out. If you want to, I'll be happy to accept PRs to use pinned package versions in CI, that will make sure this never happens again.