Assume rspec1 if RSpec::Core is not defined

This avoids wrong version detection when RSpec
is defined but rspec 1 is loaded, not rspec 2. For example when
RSpec::Instafail is defined, from rspec-instafail gem
Merge pull request #1 from nberger/detect_rspec2_with_rspec_core

Assume rspec1 if RSpec::Core is not defined