Update Travis settings for Rubinius

Apparently, since rbx-2.2.5 you no longer need to require
gem rubysl or rubinius-coverage, and you must have travis
use the ruby named 'rbx-2'.

See
*  https://github.com/tarcieri/http/pull/79
*  https://github.com/rubinius/rubinius/releases/tag/v2.2.5