stepname is in this we shouldnt hide it with an arrow function

[albertjan]

This:

.when('I look in the folder', function(next) {
  assert.ok(true, this.step);
  next();
});

instead of this:

.when('I look in the folder', (next) => {
  //this.step is undefined.
  assert.ok(true, this.step);
  next();
});

Fixes #25