Change the version constraint for composer-plugin-api

[davidbarratt]

From @Seldaek,

Heya,

I am sending you this email because you have a Composer Plugin on
https://packagist.org that requires the composer-plugin-api in
version * or 1.0.0.

Both of those are pretty dangerous:

• * would mean your plugin can get loaded in any future composer
  including 2.0, and will probably break.
• 1.0.0 means that your plugin will not be loaded anymore as soon as
  we add functionality and bump the plugin api to 1.1+

The solution is simple, open your composer.json and replace that require with:

    "composer-plugin-api": "^1.0"

Sorry for the trouble, the documentation had "1.0.0" in the plugin
example which mislead most plugin authors, but that is now fixed.

Cheers