updates for testing on ruby 2, rails 4

Dropping official support for rails 2.x, and for Ruby 1.8 + variants
Adding support for rails 4.x, and for Ruby 2.0

Testing on ruby-head is now allowed to fail. We'll continue
testing on 1.8 for now, but won't make a major effort
toward backward compatibility with 1.8.
bumping major version since we're dropping rails 2 support