Make travis automatically upload to pypi on new tags

[solarkennedy]

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[nhandler]

Ship It! But see my comment for a change we will likely want to make to improve this.

[nhandler]

I understand why you included this line, but I really don't like it. We will definitely forget to bump this version number as we change the marathon versions we test against. It also will upload to pypi even if tests are failing on older versions of Marathon (although in practice, those PRs should not be merged in the first place).

The better solution will likely be https://docs.travis-ci.com/user/build-stages , but they are still under Beta (and I'm not sure how well they work in practice).