Qt 5.6 support

[athairus]

• [x] Update Travis
• [x] Ensure msys2 packages exist before committing update to master
• [ ] Confirm our hacks around OS X HDPI support won't be hurt by native HDPI support brought in with 5.6
• [ ] Confirm #157 is fixed by this new version
• [x] QtGamepad is not included in 5.6, as far as I can tell. We'll have to wait for 5.7 for that one.
• [ ] Confirm that we're ready for the LGPLv3-only license that'll be used in 5.7 and beyond
• [x] Get hype for the Qt Quick compiler that'll become available in 5.8 (which should reduce startup times somewhat)!

Support will be done on feature-qt56, based off of master.