Fix composer script handler dependency

| Q             | A
| -------------:| ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| License       | MIT

The class `Sensio\Bundle\DistributionBundle\Composer\ScriptHandler` was added since v2.1.0 [@source](https://github.com/sensiolabs/SensioDistributionBundle/commit/aa1cf4afe0eabd01190dc4d239a57a30f851eb62)