"licenses" field is misused in package.json

[andrey-skl]

Same as https://github.com/react-component/util/issues/41:

Now "licenses" field of package.json contains single "MIT" string

According to spec, the "license" (not "licenses") field should be used to specify the single "MIT" license.

So, field should be renamed "licenses" => "license" I believe.

Thanks!