Conditional xcpretty-ing in Makefile, single build targets for travis

I don’t mean to beat a dead :horse:, but this is just a quick clean up based on the work in skabbes/mx3#11 & skabbes/mx3#18 that conditionally uses `xcpretty` when it’s available and removes the duplicate build targets for Travis.

This is just minor plumbing, so feel free to ignore.