Don't test on Ruby 2.0

Signed-off-by: David Celis <me@davidcel.is>
Test on the latest Ruby 2.1 version

Signed-off-by: David Celis <me@davidcel.is>
Use RACK_CONSOLE_INTRO, not RACK_CONSOLE_PREAMBLE

Signed-off-by: David Celis <me@davidcel.is>