Pin activeadmin to version 1.3.1

Author: depfu[bot]

activeadmin-mongoid.gemspec

@@ -19,7 +19,7 @@ Gem::Specification.new do |gem|
   gem.license       = 'MIT'
 
   gem.add_runtime_dependency 'mongoid',     ['> 3.0', '< 5.0']
-  gem.add_runtime_dependency 'activeadmin', ['>= 1.0.0.pre', '< 2']
+  gem.add_runtime_dependency 'activeadmin', ['1.3.1']
   gem.add_runtime_dependency 'jquery-rails'
   gem.add_runtime_dependency 'sass-rails',  ['>= 3.1.4', '< 5.0']
   # gem.add_runtime_dependency 'meta_search',  '~> 1.1.3'