Support activeadmin v3 (expand support version >= 2.8)

Author: shuuuuuny

activeadmin-menu_tree.gemspec

@@ -28,7 +28,7 @@ Gem::Specification.new do |spec|
   spec.executables   = spec.files.grep(%r{\Aexe/}) { |f| File.basename(f) }
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "activeadmin", "~> 2.8"
+  spec.add_dependency "activeadmin", ">= 2.8"
   spec.add_dependency "activesupport"
 
   # For more information and examples about making a new gem, checkout our