Update grunt-contrib-imagemin to 0.9.1

Upstream changes https://github.com/gruntjs/grunt-contrib-imagemin/compare/v0.4.1...v0.9.1

see #30141



git-svn-id: https://develop.svn.wordpress.org/trunk@30059 602fd350-edb4-49c9-b593-d223f7449a82