imagemin should not empty files if run repeatedly

There is an active bug in imagemin where if you run the task repeatedly it can start outputting empty files. Currently, the easiest way to get around it is to set the cache option to false.

https://github.com/gruntjs/grunt-contrib-imagemin/issues/140
Merge pull request #432 from ulisesrmzroche/master

imagemin should not empty files if run repeatedly