Replace this with grunt.task.current in changelog task (#739)

Arrow function isn't binding the context correctly here and caused the
task to fail. Keeping the nice arrow syntax we can use the current task
from the grunt object to get the `async` function.

Fix #738

Gruntfile.js

@@ -148,7 +148,7 @@ module.exports = (grunt) => {
 
 
   grunt.registerTask('changelog', '`changelog:0.0.0--0.0.2` or `changelog`', (range) => {
-    var done = this.async();
+    var done = grunt.task.current.async();
 
     if (!range) {
       // grunt changelog