Add a script so Travis can test for intentional failure

buildroot/bin/build_marlin_fail

@@ -0,0 +1,7 @@
+#!/usr/bin/env bash
+
+if arduino --verify --board arduino:avr:mega:cpu=atmega2560 Marlin/Marlin.ino ; then
+  return 1
+else
+  return 0
+fi