Add note about ActiveRecord 4.1
Add note about ActiveRecord 4.1

Concerns #33

sdolmo/sinatra-activerecord,townie/sinatra-activerecord,janko-m/sinatra-activerecord,ZalmanB/sinatra-activerecord,avillanu/uberapp,exviva/sinatra-activerecord,avillanu/uberapp,BrunaNett/sinatra-activerecord