Set mysql2 gem version

I've fixed the version of the mysql2 gem to 0.3.16. This
version works with MySQL server 5.6 without any issues.
With version 0.2.24, the database migrations fail with a
"NoMethodError: undefined method `accept' for nil:NilClass"
error

Gemfile

@@ -3,7 +3,7 @@ source 'http://rubygems.org'
 gem 'rails', '4.0.5'
 gem 'jquery-rails', '3.0.4'
 gem 'rails_autolink'
-gem 'mysql2'
+gem 'mysql2', '0.3.16'
 gem 'devise'
 gem 'twitter_oauth', git: 'git://github.com/moomerman/twitter_oauth.git'
 gem 'therubyracer'
@@ -67,4 +67,3 @@ group :test do
   gem 'ZenTest'
   gem 'database_cleaner', '~> 1.2.0'
 end
-

Gemfile.lock

@@ -390,7 +390,7 @@ DEPENDENCIES
   jquery-rails (= 3.0.4)
   json_spec
   kgio
-  mysql2
+  mysql2 (= 0.3.16)
   nested_form
   newrelic_rpm
   nokogiri
@@ -417,3 +417,6 @@ DEPENDENCIES
   webrat
   will_paginate (~> 3.0.pre2)
   will_paginate-bootstrap
+
+BUNDLED WITH
+   1.11.2