Fix warnings on mysql2 explain test

Author: carlosantoniodasilva

activerecord/test/cases/adapters/mysql2/explain_test.rb

@@ -10,15 +10,15 @@ class ExplainTest < ActiveRecord::TestCase
         def test_explain_for_one_query
           explain = Developer.where(:id => 1).explain
           assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers`  WHERE `developers`.`id` = 1), explain
-          assert_match /developers |.* const/, explain
+          assert_match %r(developers |.* const), explain
         end
 
         def test_explain_with_eager_loading
           explain = Developer.where(:id => 1).includes(:audit_logs).explain
           assert_match %(EXPLAIN for: SELECT `developers`.* FROM `developers`  WHERE `developers`.`id` = 1), explain
-          assert_match /developers |.* const/, explain
+          assert_match %r(developers |.* const), explain
           assert_match %(EXPLAIN for: SELECT `audit_logs`.* FROM `audit_logs`  WHERE `audit_logs`.`developer_id` IN (1)), explain
-          assert_match /audit_logs |.* ALL/, explain
+          assert_match %r(audit_logs |.* ALL), explain
         end
       end
     end