activeadmin 2.x support

[Mate Gabri]

I noticed the commit which introduced support for activeadmin 2.x, but if my understanding is correct, then the referenced condition on the gemspec file still locks activeadmin to 1.x.

activeadmin_dynamic_fields/activeadmin_dynamic_fields.gemspec

Line 20 in a172192

 spec.add_runtime_dependency 'activeadmin', '>= 1.0' 

[Mate Gabri]

Please ignore this, my bad. :)