Also constrain mysql2 in the rails 3.0 gemfile

See previous commit.

gemfiles/3.0.gemfile

@@ -30,9 +30,14 @@ group :development, :test do
     gem 'timecop'
   end
 
-  platforms :ruby do 
+  platforms :ruby do
     gem 'sqlite3', '~> 1.2'
-    gem 'mysql2', '~> 0.3'
+
+    # We would prefer to only constrain mysql2 to '~> 0.3',
+    # but a rails bug (https://github.com/rails/rails/issues/21544)
+    # requires us to constrain to '~> 0.3.20' for now.
+    gem 'mysql2', '~> 0.3.20'
+
     gem 'pg', '~> 0.17.1'
   end
 
@@ -42,7 +47,7 @@ group :development, :test do
     gem 'shoulda-matchers', '~> 1.5'
   end
 
-  platforms :jruby do 
+  platforms :jruby do
     # Use jRuby's sqlite3 adapter for jRuby
     gem 'activerecord-jdbcsqlite3-adapter', '~> 1.3'
     gem 'activerecord-jdbcpostgresql-adapter', '~> 1.3'