Composer\Script\CommandEvent -> Composer\Script\Event
Composer\Script\CommandEvent -> Composer\Script\Event

`Composer\Script\Event` was deprecated in composer/composer@3efed22. The
recommended signature for a script handler callback is now to use
`Composer\Script\Event` instead.

Fixes #21

thewilkybarkid/composer-merge-plugin,webflo/composer-merge-plugin,bd808/composer-merge-plugin,Priyank57/composer-merge-plugin,wikimedia/composer-merge-plugin,codeandcountry/composer-merge-plugin