(disallow|require)SpaceAfterObjectKeys: check object method shorthand (ES6)

[hzoo]

From https://github.com/dockyard/ember-suave/issues/29.

Related to #1013 since we added a check for property.method to return;

// bad
let x = {
  foo () { // space before parentheses
  }
};
// good
let x = {
  foo() {
  }
}