Merge pull request #11485 from simi/mysql2

Use latest mysql2.

Gemfile

@@ -56,9 +56,7 @@ platforms :ruby do
   group :db do
     gem 'pg', '>= 0.11.0'
     gem 'mysql', '>= 2.9.0'
-
-    # TODO: Relax dependency version after of fix #11457
-    gem 'mysql2', '0.3.11'
+    gem 'mysql2', '>= 0.3.13'
   end
 end