Remove rbx from Travis

The tests will not run on rbx 2.0 in 1.9 mode yet. I keep getting this error:

    1) RR::Space#singleton_method_added_injection when a SingletonMethodAddedInjection is not registered for the subject and method_name overrides the method when passing a block
       Failure/Error: expect(subject.method(:singleton_method_added)).to_not eq original_method
       NoMethodError:
         undefined method `block_env' on an instance of Rubinius::CompiledCode.
       # kernel/delta/kernel.rb:81:in `block_env (method_missing)'
       # kernel/common/block_environment.rb:128:in `=='
       # kernel/common/method19.rb:15:in `=='
       # ./spec/suites/rspec/unit/space_spec.rb:168:in `RR'
       # kernel/common/eval19.rb:45:in `instance_eval'
       # kernel/bootstrap/array19.rb:18:in `map'
       # kernel/bootstrap/array19.rb:18:in `map'
       # kernel/bootstrap/array19.rb:18:in `map'
       # kernel/bootstrap/array19.rb:18:in `map'
       # kernel/bootstrap/proc.rb:22:in `call'
       # kernel/loader.rb:697:in `run_at_exits'
       # kernel/loader.rb:717:in `epilogue'
       # kernel/loader.rb:850:in `main'

I think this is fixed in recent versions of Rubinius but the rbx testing branch
is not up to date with the latest changes yet.