mysql gem no longer used.

git-svn-id: http://svn.redmine.org/redmine@13293 e93f8b46-1217-0410-a6f0-8f06a7374b81
Reverts r13279.

uninitialized constant Rake::TestTask error: bundle is not yet completed when running rake -f lib/tasks/ci.rake config/database.yml on the CI server. What was the problem with Rake::Task["test"].invoke ?

git-svn-id: http://svn.redmine.org/redmine@13294 e93f8b46-1217-0410-a6f0-8f06a7374b81