Cache Organisations for an hour

Currently, organisations are cached in each process until the app is restarted. So new and changed organisations aren't reflected in the app, possibly for months.

This change means they are only cached for an hour when fetched from govuk_need_api. govuk_need_api only updates once per day from Whitehall, but I'm going to change this in a separate pull request. Speaking with @fatbusinessman, Maslow uses govuk_need_api for organisations to avoid a dependency on Whitehall in development.

Copied from smart-answers WorldLocation model, hat tip to @alext.
This conditional should be unnecessary.  `LRUCache#fetch` takes a block which is called if the value isn't in the cache, and then stores and returns the result.  This could therefore be rewritten as:
```ruby
value = cache.fetch(key) do
  yield
end
```

Admitedly the [docs for fetch](http://rubydoc.info/gems/lrucache/LRUCache#fetch-instance_method) are non-existent, but if you look at the source, you can see that this is the case.