Bump Rails to 4.1.7.1

This is a security fix update (though we don't believe we were vulnerable).

https://github.com/alphagov/email-alert-api/pull/54 attempted to update
rails to 4.1.8, but that release included other changes which caused our
build to break (due to problems setting up the postgres hstore
extension).

4.1.7.1 was then released, containing just the security fixes.