Support for jQuery siblings function; a fix for the 'on' event handler function arguments; a fix for a broken spec

This is a companion pull request to https://github.com/opal/opal-jquery/pull/11.  See that request for details.  The opal-jquery.js file included here was generated from the opal-jquery build task.

Also,  Capybara wasn't being included on specs, so the 'visit' method wasn't found and a spec failed.  That's fixed here, too.