travis: disable nightly build due to class name \yii\base\Object conflicting with new keyword

[...]
  - Installing yiisoft/yii2-composer (2.0.5): Downloading (100%)
[...]
PHP Fatal error:  Cannot use 'Object' as class name as it is reserved in /home/travis/build/fphammerle/yii2-client-cert-auth/vendor/yiisoft/yii2/base/Object.php on line 77
[...]

.travis.yml

@@ -5,7 +5,7 @@ php:
   - '5.5'
   - '5.6'
   - '7.0'
-  - nightly
+#  - nightly
 
 install:
 - composer global require "fxp/composer-asset-plugin:^1.3.1"