Gemfile tidy

This does two things:

1. Removes some unused/unnecessary gems
2. Updates a lot of the outdated gems.

The major change is with capyabara, which behaves slightly differently in the latest version (see https://github.com/alphagov/whitehall/commit/caae97573aa2f22184ee394edac629c1ce332116).

A couple of major gems have not been updated:

1. carrierwave - I know @matthewford was having some issues with this in his 3.2 update, so I thought it best to leave this as-is until that is merged.
2. transistions - there appears to be a bug in the latest version that only affects <3.2. Once we are running on 3.2, we should be able to bump this.
3. whenever - the changelog suggests some changes that may or may not have an impact on deployment so I figured this should be updated in isolation to make it easier to unpick if there are issues.