Run tests on Windows via Travis CI

[clue]

This PR adds Windows to the test matrix on Travis CI. Most of this project should work cross-platform, but we've added some Windows-specifics with #67, so we have reason to believe that tests help us ensure we do not introduce any regressions in the future.

Windows platform tests are currently allowed to fail, given how we've tried to add similar tests in the past (#21, #28 and #70) and also given that Windows platform support on Travis is currently considered "early release" (https://blog.travis-ci.com/2018-10-11-windows-early-release) and the test setup contains some workarounds. I consider this to be a first step and there's hope we can build on top of this in the future 👍

Credit where credit is due: Thanks @Lekensteyn for the detailed post in https://travis-ci.community/t/feedback-from-windows-integration-for-a-cmake-qt-c-python-perl-project/1706 which helped me immensely (spawning a powershell to read the updated PATH environment).

Builds on top of #67
Refs reactphp/stream#120 and reactphp/stream#112

[Lekensteyn]

You might want to add --no-progress to choco install, this reduces the amount of spam in the full log file. The download is still much smaller than Travis' timeout, so it's no problem to remain silent while downloading.

[clue]

You might want to add --no-progress to choco install, this reduces the amount of spam in the full log file.

@Lekensteyn Thanks for chiming in, I agree that this might be an option. This output is part of the "installation" step and Travis does a decent job at collapsing this by default, so I don't think this is an issue currently.