Bump simplekv minimal version

[jacquerie]

Closes #43, in particular see #43 (comment): it would be cool to have a new release after this commit.

[jacquerie]

I prepended another commit that fixes the build, as apparently it was broken, by switching to tox-travis. I got this solution from https://stackoverflow.com/a/37062119.

[jacquerie]

@mbr I think it's ready. Whenever you have time 👍

[lnielsen]

@mbr Any news on if this can be integrated and released. If you like, we'd be happy to help maintain the package.

[mbr]

I've recently handed over maintainership of simplekv; now it is slightly more work to ensure both packages are compatible. @jacquerie @lnielsen If you're interesting in helping out maintain this, please write me an email to talk about this =).