Add Python 3.7 to the build matrix

[alexwlchan]

Resolves #1148.

It's quite possible (maybe likely!) this won't work in its current state, but let's get it into Travis and see what happens.

[Zac-HD]

@alexwlchan - the culprit looks like scripts/install.sh to me - there's no 3.7 entry, and it's somehow defaulting to 3.6 (yay bash!).

Latest version at time of writing is 3.7.0b3, from https://www.python.org/download/pre-releases , and pyenv should handle that string as a version to install just fine.

(I'm assuming here that you've left this open with the intention of further yak shaving)

[alexwlchan]

Thanks @Zac-HD. I’d left it open because I’d forgotten about it, TBH, but I’ve had another go at getting it working.

[Zac-HD]

The easiest way forward for this is probably to wait for #1227, and build it on top of the new runner.