Fix unsupported flag in php 5.3.x environment [ArrayHelper::multisort]

The use of `SORT_NATURAL` flag, causing the [same trouble](https://travis-ci.org/toopay/yii2/jobs/6890963#L37) we have in #7 and #93. Since we're already add case-insensitive flag internally within the method, i think its save to remove completely any php 5.4-only constants.