Bump mysql gem version to the newly 2.9.0, fix build.

Gemfile

@@ -51,7 +51,7 @@ platforms :ruby do
 
   group :db do
     gem 'pg', '>= 0.11.0'
-    gem 'mysql', '>= 2.8.1'
+    gem 'mysql', '>= 2.9.0'
     gem 'mysql2', '>= 0.3.10'
   end
 end

activerecord/lib/active_record/connection_adapters/mysql_adapter.rb

@@ -2,7 +2,7 @@
 require 'active_record/connection_adapters/statement_pool'
 require 'active_support/core_ext/hash/keys'
 
-gem 'mysql', '~> 2.8.1'
+gem 'mysql', '~> 2.9.0'
 require 'mysql'
 
 class Mysql