travis: disable Rubinius again

Because Rake is failed on Rubinius:

    $ bundle exec rake
    An exception occurred running /home/travis/.rvm/gems/rbx-2.2.1@global/bin/rake:

        no such file to load -- singleton (LoadError)

    Backtrace:

           Rubinius::CodeLoader#load_error at kernel/common/code_loader.rb:440
      Rubinius::CodeLoader#resolve_require_path at kernel/common/code_loader.rb:423
       { } in Rubinius::CodeLoader#require at kernel/common/code_loader.rb:103
                      Rubinius.synchronize at kernel/bootstrap/rubinius.rb:137
              Rubinius::CodeLoader#require at kernel/common/code_loader.rb:102
              Rubinius::CodeLoader.require at kernel/common/code_loader.rb:237
                    Kernel(Object)#require at kernel/common/kernel.rb:685
                        Object#__script__ at /home/travis/.rvm/gems/rbx-2.2.1@global
                                             /gems/rake-10.1.0/lib/rake.rb:32
              Rubinius::CodeLoader.require at kernel/common/code_loader.rb:243
                    Kernel(Object)#require at kernel/common/kernel.rb:685
                        Object#__script__ at /home/travis/.rvm/gems/rbx-2.2.1@global
                                             /gems/rake-10.1.0/bin/rake:31
                       Kernel(Object)#load at kernel/common/kernel.rb:427
                        Object#__script__ at /home/travis/.rvm/gems/rbx-2.2.1@global
                                             /bin/rake:19
          Rubinius::CodeLoader#load_script at kernel/delta/code_loader.rb:66
          Rubinius::CodeLoader.load_script at kernel/delta/code_loader.rb:198
                   Rubinius::Loader#script at kernel/loader.rb:649
                     Rubinius::Loader#main at kernel/loader.rb:831

    The command "bundle exec rake" exited with 1.

https://travis-ci.org/ruby-gettext/gettext/jobs/15520603