Allow Rails 5.2 in gemspec
Allow Rails 5.2 in gemspec

Support of all releases in the 5.2.x branch, but avoid similar issues
like those mentioned in #4916.

mauriciopasquier/active_admin,mrjman/activeadmin,activeadmin/activeadmin,vraravam/activeadmin,activeadmin/activeadmin,activeadmin/activeadmin,vraravam/activeadmin,vraravam/activeadmin,deivid-rodriguez/activeadmin,javierjulio/activeadmin,deivid-rodriguez/activeadmin,javierjulio/activeadmin,mauriciopasquier/active_admin,mrjman/activeadmin,deivid-rodriguez/activeadmin,javierjulio/activeadmin,mrjman/activeadmin,mauriciopasquier/active_admin