Remove empty template

I'm rendering the bookmark as json directly from the controller so this
template is unnecessary.
Allow requests to the json endpoint from anywhere

I *think* this is what I need to do to avoid an error similar to "Origin
<origin-website> is not allowed by Access-Control-Allow-Origin."
Add Javascript example to README
Update gems

I ran `bundle update` to get the latest version of all the bundled gems.

I had to fix up a couple of tests as the new Capybara behaviour is to raise an
Ambiguous match error if multiple links are found with the same text. I
followed the advice of using `first(:link, 'text').click` (seen in the upgrade
guide[1]) for one of the tests, and changed the template to make the link text
more specific for the second.

[1] Edit the bookmark for http://foo.com