Fall back on Test::Unit::Assertions if rspec-expectations is missing

Currently, users who don't want to use rspec-expectations have to do a little extra setup in their env.rb to use Test::Unit.  Since Test::Unit ships with Ruby (albeit as a wrapper in 1.9), it makes sense to look
for this library as a fallback.  This lowers the barrier even further for users who want to jump straight into Cucumber with minimal dependencies.