Revert "disables the caching functionality of imagemin transitionally un...

...til the problem with the current version of grunt-contrib-imagemin (0.5.0) is sorted out"

This reverts commit 3e435fa74b1574223f129867621a9a800cea2af9.

The problem doesn't seem to occur any longer.

... would like to now what change fixed it - as the dependency is still on grunt-contrib-imagemin (0.5.0)