Attempt to fix the travis build for rubinius

Rubinius no longer supplies 1.8-mode or 1.9-mode, and they have changed
*how* modes are to be selected entirely, along witht he 2.x breaking
change to extract the ruby standard library into the gem 'rubysl'.

Effectively this means that we can no longer verify support for 1.8 mode
or 1.9 mode, and that we must add the rubysl gem as a platform-specific
dependency to the Gemfile and require that consumers of redis-namespace
on rubinius also have this gem on their load path.