Downgrade Rubinius's test/unit gem to avoid a problem with the latest

See comment for details. The exception looks like this:

    MiniTest::Unit::TestCase is now Minitest::Test. From /Users/chrisk/.rvm/gems
    /rbx-2.2.1/gems/rubysl-test-unit-2.0.2/lib/test/unit/testcase.rb:8:in
    `__script__'
    An exception occurred running /Users/chrisk/.rvm/gems/rbx-2.2.1@global/gems
    /rake-10.1.0/lib/rake/rake_test_loader.rb:
    undefined method `_run_suite' for class `Test::Unit::Runner' (NameError)

    Backtrace:

    Module(Class)#alias_method at kernel/delta/module.rb:39
    Rubinius::ConstantScope#alias_method at kernel/common/constant_scope.rb:93
    { } in Object(Class)#__script__ at /Users/chrisk/.rvm/gems/rbx-2.2.1/gems/
                       rubysl-test-unit-2.0.2/lib/rubysl/test/unit/unit.rb:676
    ...