Update gemspec authors
Update gemspec authors

Include current maintainers and Piers.

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