Windows builds

[TheSpyder]

This extends #73, when I realised travis was running against my PR I decided to take a crack at using travis to build on windows.

It works! Fixes #20.

[TheSpyder]

Travis windows VMs don't support secure secrets very well 😭

I thought the build was hanging, so I temporarily deleted the deploy section, but it runs it just doesn't stream the live build info. However there is a risk of hanging. The only way to fix this is to disable filter_secrets, which runs the risk of secrets leaking: https://travis-ci.community/t/current-known-issues-please-read-this-before-posting-a-new-topic/264/10

From the looks of those docs anyone could set filter_secrets to false today and create a PR to echo the secret, so maybe it's not as much of a risk as it sounds.

I don't want to disable filter_secrets without approval, although it can be disabled only on windows, so for the moment this PR will just have a caveat that windows build progress can't be reported.

[TheSpyder]

Two things are currently failing

• I didn't add dev mode delegation to the ppx.cmd windows binary, it didn't seem necessary, but of course it is necessary to build the tests
• Moving bin.exe into the correct place failed, I'm not sure if that's because mv doesn't exist or the $TRAVIS_OS_NAME variable doesn't exist. I'll resurrect my windows build environment next week, I don't want to keep spamming travis with builds (or maybe I can just disable osx/linux and run windows build experiments that way).

[TheSpyder]

@ryb73 this is good to go!