Allow sudo to mitigate the Rubinius failure

Even though forbidding sudo to be used would allow us to run the Travis
build on the new container-based infrastructure, this makes the Rubinius
build to fail as it looks like RVM needs the sudo command to install
Rubinius.

Cross-refs #64.
Merge pull request #66 from robin850/fix-rbx-build

Allow sudo to mitigate the Rubinius failure