Set RBENV_ROOT

[trkoch]

Otherwise rbenv init initializes with default location, i.e. gem
binaries cannot be found.

[trkoch]

A temporary workaround is to set RBENV_ROOT in the environment task, right before loading rbenv, like this:

task :environment do
  queue %{export RBENV_ROOT="#{rbenv_path}"}
  invoke :'rbenv:load'
end

[rstacruz]

Good catch!