Add support for ActiveAdmin version 3

[dssjoblom]

This gem works fine with AA 3, but it is disallowed by the gemspec. This commit fixes that.

[varyonic]

I propose dropping the version constraint, I don't think it adds value. Even better drop the dependency altogether(!), I'm using this with a project fork. I doubt anyone is specifying activeadmin-selectize in a Gemfile without already having activeadmin present.