Bump mysql2 to 0.3.20

Keep it locked at the 0.3.x version due to problems with other apps and
the 0.4.x versions.

Gemfile

@@ -21,7 +21,7 @@ gem 'jquery-rails'
 # always possible on all platforms
 # Alternatively use --without <group> arguments to bundler to not install that group
 gem "sqlite3", group: :sqlite
-gem "mysql2", group: :mysql
+gem "mysql2", "~> 0.3.17", group: :mysql
 
 gem "RedCloth"
 gem "sanitize", ">=3.0.0"

Gemfile.lock

@@ -118,7 +118,7 @@ GEM
       metaclass (~> 0.0.1)
     multi_json (1.11.1)
     multi_test (0.1.1)
-    mysql2 (0.3.16)
+    mysql2 (0.3.20)
     nokogiri (1.6.4)
       mini_portile (~> 0.6.0)
     nokogumbo (1.1.12)
@@ -245,7 +245,7 @@ DEPENDENCIES
   htmlentities
   jquery-rails
   mocha
-  mysql2
+  mysql2 (~> 0.3.17)
   paperclip
   rack-dev-mark
   rack-mini-profiler