Remove over-zealous HTTP status assertions

We're already asserting this for the same slug. No need to repeat the
assertion. If it's anything other than a 200, Rails won't be serving
up the view anyway.
We're already passing in jQuery so can call it with $

The function is being invoked with jQuery as a parameter so we know to
expect the $ object.
Do not refresh the page when hitting enter in the country filter

This fixes a bug where the page would reload. We're just catching the
default behaviour that would occur for users with this script.
Merge pull request #277 from alphagov/do-not-refresh-search-for-js-filter

Do not refresh the page when hitting enter in the country filter