How can I execute a command and use it's return value while deploying with Capistrano 3?

Question

the title pretty much says it all. I upgraded from Cap 2 to Cap 3 and this command doesn't work anymore:

set :whenever_command, "bundle exec whenever -f #{File.join("\`bundle show my_gem\`", "config", "schedule.rb")}"

bundle showis not executed and returns the string itself.

How can I do this with Capistrano 3?