remove update-shrinkwrap from the default Grunt task

Most folks shouldn't care about the shrinkwrap.
It should only be run on-demand, not as a routine/periodic thing.

Gruntfile.js

@@ -434,7 +434,7 @@ module.exports = function (grunt) {
   grunt.registerTask('dist', ['clean', 'dist-css', 'copy:fonts', 'dist-js', 'dist-docs']);
 
   // Default task.
-  grunt.registerTask('default', ['test', 'dist', 'build-glyphicons-data', 'build-customizer', 'update-shrinkwrap']);
+  grunt.registerTask('default', ['test', 'dist', 'build-glyphicons-data', 'build-customizer']);
 
   // Version numbering task.
   // grunt change-version-number --oldver=A.B.C --newver=X.Y.Z