Updated SKR Pro Build

This build contains no code changes, however the binary is built using the gcc 6.3.1 toolchain. This seems to resolve issues some have with the bootloader not enumerating USB.

Please keep in mind that after programming the bootloader you need to manually enter the it by setting the BOOT1 pin high with a jumper. After the application has flashed remove power, remove then jumper, then power on the device. The application will then load.