Build/Test Tools: Load grunt-contrib-imagemin again.

In [59797], the `matchdep` dependency was removed because it has been abandoned. The `grunt-contrib-imagemin` package was missed in the list of `gruntDependencies` to load that replaced the functionality for `matchdep`.

This adds `imagemin` to the list to restore the `precommit:image` Grunt task functionality.

Props jorbin.
Fixes #64328.

git-svn-id: https://develop.svn.wordpress.org/trunk@61664 602fd350-edb4-49c9-b593-d223f7449a82

Author: desrosj

Gruntfile.js

@@ -107,6 +107,7 @@ module.exports = function(grunt) {
 			'concat',
 			'copy',
 			'cssmin',
+			'imagemin',
 			'jshint',
 			'qunit',
 			'uglify',