loosen activeadmin restriction to allow 2.0

Author: iarie

active_admin_datetimepicker.gemspec

@@ -19,6 +19,6 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{^bin/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activeadmin", "~> 1.1"
+  spec.add_dependency "activeadmin", ">= 1.1", "< 3.x"
   spec.add_dependency "xdan-datetimepicker-rails", "~> 2.5.4"
 end