build: Use conf.MessageDirs for banana path config

Rather than configuring manually, re-use the specification in the
repo's extension.json file, like we do elsewhere; this way, if we
add further i18n files in future, or change their paths, we won't
need to update this configuration.

Change-Id: I4fec61599b1a8b9ac9de93983dfbc2e0dfa97286

Gruntfile.js

@@ -1,5 +1,7 @@
 /* eslint-env node, es6 */
 module.exports = function ( grunt ) {
+	var conf = grunt.file.readJSON( 'extension.json' );
+
 	grunt.loadNpmTasks( 'grunt-eslint' );
 	grunt.loadNpmTasks( 'grunt-banana-checker' );
 
@@ -14,9 +16,7 @@ module.exports = function ( grunt ) {
 				'!vendor/**'
 			]
 		},
-		banana: {
-			all: 'i18n/'
-		}
+		banana: conf.MessagesDirs
 	} );
 
 	grunt.registerTask( 'test', [ 'eslint', 'banana' ] );