Eleventy: Add Windows to travis-ci builds

[zachleat]

https://blog.travis-ci.com/2018-10-11-windows-early-release

[zachleat]

Windows samples here https://travis-ci.org/11ty/eleventy/builds/478638999

[zachleat]

Make sure https://github.com/11ty/eleventy/pull/381#issuecomment-453758111 is working before this is added.

[kleinfreund]

Maybe we could do something like sorting both expected and actual results.

[zachleat]

Tried this again, looks a bit more straightforward this time: https://travis-ci.org/11ty/eleventy/builds/516470666

[kleinfreund]

The errors are related to the url() function which has some code producing Windows-style path delimiters. This should never happen when dealing with URLs. Should be easy enough to fix. Unfortunately, I no longer have a Windows system available to me to test this. Maybe a fix is possible without by looking at the Travis results.

[zachleat]

Looks like this was fixed in #486! Thank you @AndrewAsquith!!

[zachleat]

Heck yeah https://travis-ci.org/11ty/eleventy/builds/518844789

[kleinfreund]

Heck yeah indeed!