Fixed transparent background for ImageMagick in CCaptchaAction

thanks to @manuel-84
https://github.com/yiisoft/yii/pull/1464#issuecomment-12046268
Merge branch 'master' of https://github.com/yiisoft/yii

* 'master' of https://github.com/yiisoft/yii:
  Fixes #1867 Gii model generator tableName autocomplete