Simply overwrite the job_type to include the rvm version you want

job_type :rake, "source /etc/profile.d/rvm.sh && rvm use ruby-2.0.0-p247 && cd :path && :environment_variable=:environment bundle exec rake :task --silent :output"

every 1.day, :at => '8:00 am' do
  rake "users:activities"
end