Improve how we detect module prepend support.

JRuby 1.7.4+ has experimental ruby 2.0 support,
and reports `RUBY_VERSION` as 2.0 when run with
`JRUBY_OPTS='--2.0'` but does not support module
prepends.

Fixes rspec/rspec-mocks#672.
Merge pull request #66 from rspec/fix-prepend-detection

Improve how we detect module prepend support.