Upgrade rails

Really the work of yesteday's  work. Somebody fancy checking?
Maybe, but then: why? The exact version is locked anyway in
Gemfile.lock. If someone runs 'bundle update rails', s/he would need to
do whatever is necessary to get it to work with whatever the latest
version at that point then is. I don't see a reason why we would want to
specify here that we never want to go above rails 4.1 - if the app works
with a later version, why not?