Fix rspec deprecation warning on arguments to #raise_error.

The deprecation warning was:

DEPRECATION: `expect { }.not_to raise_error(SpecificErrorClass, message)` is
deprecated. Use `expect { }.not_to raise_error` (with no args) instead. Called
from /source/travis-ci/travis-lint/spec/travis_lint_runner_spec.rb:93:in
`__script__'.
Update specs for rbx-1[89]mode being invalid.
Update validators for rbx. Fixes #36.
Remove rbx-1[89]mode from .travis.yml.
Add gem 'rubysl' to Gemfile for Rubinius.