Update Travis config (#36)
Update Travis config (#36)

dist:

    - remove `dist: trusty` as that's the default:
      https://blog.travis-ci.com/2017-07-11-trusty-as-default-linux-is-coming
      https://blog.travis-ci.com/2017-08-31-trusty-as-default-status

Rubinius:

    - use the recommended Rubinius version (rbx-3):
      https://github.com/travis-ci/docs-travis-ci-com/pull/1534

    - allow Rubinius tests to fail due to Travis/RVM issues and
      incompatibility with Rake:
      https://github.com/vmg/redcarpet/issues/580
      https://github.com/ruby/rake/pull/150

MRI:

    - remove end-of-life Ruby versions:
      https://www.ruby-lang.org/en/downloads/branches/
bbatsov/powerpack