Revert "Tests now require MySQL2 adapter in order to run"

Reason: Setting up MySQL and Postgres needs installing servers
and libraries that perhaps you don't need in your machine. We
think fixing a bug in Active Support without running the suite
of Active Record for all possible drivers is OK. That's why
we explain first --without db, and after that we cover AR
specifically. mysql2 is no different than mysql or pg in that
sense.

This reverts commit 50d5e0f7e8.

railties/guides/source/contributing_to_ruby_on_rails.textile

@@ -81,10 +81,10 @@ $ gem install bundler
 and run:
 
 <shell>
-$ bundle install
+$ bundle install --without db
 </shell>
 
-This command will install all dependencies. With dependencies installed, you can run the test suite with:
+This command will install all dependencies except the MySQL and PostgreSQL Ruby drivers. We will come back at these soon. With dependencies installed, you can run the test suite with:
 
 <shell>
 $ rake test
@@ -139,6 +139,15 @@ $ sudo apt-get install mysql-server libmysqlclient15-dev
 $ sudo apt-get install postgresql postgresql-client postgresql-contrib libpq-dev
 </shell>
 
+After that run:
+
+<shell>
+$ rm .bundle/config
+$ bundle install
+</shell>
+
+We need first to delete +.bundle/config+ because Bundler remembers in that file that we didn't want to install the "db" group (alternatively you can edit the file).
+
 In order to be able to run the test suite against MySQL you need to create a user named +rails+ with privileges on the test databases:
 
 <shell>