Don't use `%zx` because it's unsupported on mingw64 (#578)

* Don't use `%zx` because it's unsupported on mingw64
* Also check that no warnings happen on travis
* Prefer `unsigned int` over `unsigned`

* Install libusb through the travis whitelist instead of manually

* Test stlink against clang and gcc through the travis "compiler" flag instead of a manual matrix

* Re-add osx

* Fix warning on gcc