Upgrade to ember-cli-babel@^6.0.0

This addon is outdated with the recent Ember versions

```
ember-cli-clock@2.1.1:
  version "2.1.1"
  resolved "https://registry.yarnpkg.com/ember-cli-clock/-/ember-cli-clock-2.1.1.tgz#342d18f14eddc855dd91d448e85296d6a2482077"
  dependencies:
    ember-cli-babel "^5.1.5"
```

```
$> ember -v

ember-cli: 2.14.1
node: 8.2.1
os: darwin x64
```