Grunt: switch imagemin back to version 0.4.1

0.6.1 has some dependency issue that doesn’t seem to be automatically
resolved. Switching it back to the version of the package that
WordPress core uses.

This commit is part of a series that will re-organise BuddyPress trunk
to use a Grunt build process. See #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8373 cdf35c40-ae34-48e0-9cc9-0c9da1808c22
Grunt: run the `grunt imagemin` task.

Reduces size of images by 15.77 kB.

This commit is part of a series that will re-organise BuddyPress trunk
to use a Grunt build process. See #5160

git-svn-id: https://buddypress.svn.wordpress.org/trunk@8374 cdf35c40-ae34-48e0-9cc9-0c9da1808c22