Travis CI requires a version to be appended to :

> rbx is not installed - installing.
> Searching for binary rubies, this might take some time.
> Requested binary installation but no rubies are available to download, consider skipping --binary flag.
> Gemset '' does not exist, 'rvm rbx do rvm gemset create ' first, or append '--create'.
> The command 'rvm use rbx --install --binary --fuzzy' failed and exited with 2 during setup.