Fix composer autoload script

Author: schmengler

.travis/before_script.sh

@@ -35,7 +35,7 @@ fi
 # Add tests/src to autoload-dev on project level
 php -r '$composer_json = json_decode(file_get_contents("composer.json"), true);
 $composer_json["autoload-dev"]["psr-4"]["IntegerNet\\RewriteMap\\"] = "vendor/integer-net/magento2-rewritemap/tests/src";
-file_put_contents("composer.json", $composer_json);'
+file_put_contents("composer.json", json_encode($composer_json));'
 composer dumpautoload
 
 # prepare for test suite