Fix Invalid Operation (+) error when env vars are missing.

HOMEDRIVE/HOMEPATH are not always guaranteed to be present, if they are not, haxelib throws "Invalid Operation (+)".
http://stackoverflow.com/questions/18909393/trying-to-install-openfl-haxelib-returns-invalid-operation

As a more permanent solution to this, should we store haxelib config data in AppData? This is the correct location for it. If not, could we switch to USERPROFILE?