Convert xcode_dev to unicode before building sysroot

This addresses an issue (#1837) when installing Kivy (1.8.0) on OSX using Python 3.x

[Full traceback at http://pastebin.com/rWyrb30q](http://pastebin.com/rWyrb30q)

The problem arises when attempting to join a bytecode string with unicode strings.

This PR converts the bytecode to unicode before the join operation is attempted.