Travis improvements

[JonnyPtn]

Refines the script a little bit and adds some functionality:

• Windows builds included
• iOS builds included (should work once #40 is merged, but can be removed if you'd prefer)
• Named build matrices for clarity
• General tidy up

example output here: https://travis-ci.org/JonnyPtn/bgfx.cmake

edit: Also notice you don't have your travis account integrated with github so the checks don't show up in the github UI - is this intentional?

[JoshuaBrookover]

Thanks so much. It's been on my todo but this goes above and beyond.