Add support for release candidates

[kkiningh]

Resolves issue #10. Might want to refactor a tiny bit so that upgrading the release candidate means only changing the version string and not the url.

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[kkiningh]

I signed it!

[googlebot]

CLAs look good, thanks!

[kkiningh]

Is the build failure related to my pull request? If there's something I should change let me know.

[philwo]

Hi @kkiningh,

thanks, that looks pretty good! Don't worry about the test - I think our current Travis setup is a bit broken. I'll ping this PR when it should work again, so we can rerun the tests on this one (although I don't see any issues).

[philwo]

@kkiningh Sorry for the delay in merging :( Would you mind rebasing your PR? I'll merge it then! Otherwise I could also just apply your proposed changes to the current file myself (but then it wouldn't be in your name, so I wanted to check). Let me know. :)

[kkiningh]

Feel free to apply the changes yourself! I'm on vacation and won't be able to get to it until after Christmas.