Update the version of Semantic UI

[jamesmacfie]

The version of Semantic UI that we are using doesn't allow the addition of responsiveness very easily so updating the version would be a good call. Adding a 'responsive' navigation to the current site proved rather hacky.

That said, if anyone thinks that there's no reason to use a CSS framework at all or even a (smaller?) alternative to Semantic UI then that could be an option too.