travis: Use return instread of exit in stage_script

https://docs.travis-ci.com/user/customizing-the-build#How-does-this-work%3F-(Or%2C-why-you-should-not-use-exit-in-build-steps)

.travis/stage_script.sh

@@ -7,7 +7,7 @@ if [ "$RUN_PYLINT" ]; then
     .travis/check_pylint_diff
     EXIT_CODE=$?
     echo "Lint check returned ${EXIT_CODE}"
-    exit ${EXIT_CODE}
+    return ${EXIT_CODE}
 fi
 
 cd "$ORANGE_DIR"