Don't ask Chrome 34 users to upgrade

We still get a lot of usage for it, and it's not _that_ old. Err on the side of not annoying users
Increase the browser upgrade dismissal duration

As some users are not in a position to upgrade, eg, anywhere with restrictive IT policies and slow update rollout.

While we still want to re-remind users, we shouldn't do it too often. A month seems reasonable
Merge pull request #423 from alphagov/browser-upgrade-behaviour-tweak

Make legacy browsing checking less aggressive