Update rails to 3.1 and remove condition on sprockets version.
Update plugin 'http_accept_language' to version that supports Rails 3.1.
Install 'rails_legacy_mapper' so we don't have to update all routes.
This should be temporary.
Update Session configuration.
Update path configuration.
Load the correct sass file.
debug_rjs was removed.
ActiveRecord::Associations::AssociationProxy was split into multiple classes.
Use ActiveRecord::Associations::CollectionProxy.
RAILS_DEFAULT_LOGGER is not defined anymore.
Install 'prototype-rails' in order to be able to use prototype.
The source of a has_one :through association can't be a has_many association.
Fake it instead.
Use [] instead of #on for errors.
Fix setter for private wiki.
Use an instance method to return the peers of a user instead of an association.
try also running travis with ruby 2.0.0 and 2.1.1
upgrade mysql2 gem to a version working with rails 3.1
turn of session cookies the new way

Haven't tested this. more options can be found here:
https://stackoverflow.com/questions/5435494/rails-3-disabling-session-cookies

Not touching the session at all should also work as it is lazy loaded.
replace RAILS_ROOT
upgrade url_for stubbing to rails 3.1 style
move session store options into application config

The initializer is deprecated since rails 3.