Fix requirements in README
Fix requirements in README

Also, remove old migration explanations and split getting started in two
sections, one for Rails and other for Rack/Sinatra

trevorturk/exception_notification,trevorturk/exception_notification,Axxiss/exception_notification,trevorturk/exception_notification,Axxiss/exception_notification,Aschen/exception_notification,DennisSchmidt/exception_notification,Axxiss/exception_notification,Aschen/exception_notification,DennisSchmidt/exception_notification,smartinez87/exception_notification,DennisSchmidt/exception_notification,smartinez87/exception_notification,Aschen/exception_notification