Add travis support for OS X tests and PHP 7.3

[beryllium]

Travis now has experimental support for testing under OS X (and Windows, but that doesn't seem to play nicely with PHP yet). Maybe it will help Sculpin to try and test under these common user envs.

Technically their Windows support doesn't include PHP, and their OS X support may not support it either. Let's find out. (Update: discovered that yes, OS X can be done, but Windows is still difficult.)

[beryllium]

Well, I was able to get OS X to work, but Windows will be a tough nut to crack. Looks like this guide from jeff geerling might help for manual testing, but could be difficult to follow programmatically inside Travis:

https://www.jeffgeerling.com/blog/2018/installing-php-7-and-composer-on-windows-10

[lex111]

Apparently, now we can not integrate Windows:

The language 'ruby' is currently unsupported on the Windows Build Environment.

By the way, can migrate to PHP 7.3?

[beryllium]

I don't think we should move the composer.json requirements to 7.3 for this version, since 7.2 is still the default in some widely-available systems. I've added 7.3 support to travis, though, which should help ensure some stability there.

Even though this PR is about Windows, I had significant difficulty getting it to build under Travis. I was able to test it out locally using a Windows VM and everything seemed to work.

[WyriHaximus]

@beryllium have you consired AppVeyor for Windows?

[WyriHaximus]

This simplefies my own set up so much, thank you 👍 .

[lex111]

👏

[lex111]

You can also consider using GitLab CI...