Improved the JRuby check

- ENV['RUBY_VERSION'] under JRuby will be something like "ruby-1.9.3-p327".
- JRUBY_VERSION will be defined and something like "1.7.2".
- I would write the SequelRails.jruby? method without memoization (because there
  are no performance ramifications) as a one-liner, but it's a style preference:

  (defined?(RUBY_ENGINE) && RUBY_ENGINE == "jruby") || defined?(JRUBY_VERSION)
Merge pull request #23 from edruder/improve-check-for-jruby

Improved the JRuby check