Add a comment about the need for the composer hack

Author: redthor

.travis.yml

@@ -19,6 +19,7 @@ before_script:
   # Remove xdebug at the start
   - phpenv config-rm xdebug.ini
   - pecl install -f mongodb-stable
+  # Until doctrine/mongo switches to mongodb driver
   - composer config "platform.ext-mongo" "1.6.16"
   - composer self-update
   # To be removed when this issue is resolved: https://github.com/composer/composer/issues/5355