Fixed return code 0 for 'mbed --version'

Author: screamerbg

mbed/mbed.py

@@ -2221,7 +2221,7 @@ def help_():
 
 if '--version' in sys.argv:
     print ver
-    sys.exit(1)
+    sys.exit(0)
 
 pargs, remainder = parser.parse_known_args()
 status = 1