build: stylelint now ignores /vendor/

When using composer install at the root of the skin, stylelint attempts
to lint all the PHP packages and report errors.  Ignore the directory
entirely like we do for jshint or jsonlint.

Change-Id: I327ae63024f5c53be628297a438187ff91dfa962

Gruntfile.js

@@ -29,7 +29,8 @@ module.exports = function ( grunt ) {
 				'**/*.css',
 				'**/*.less',
 				'!resources/libraries/**',
-				'!node_modules/**'
+				'!node_modules/**',
+				'!vendor/**'
 			]
 		}
 	} );