Fix capistrano v3 roles

Capistrano roles are broken in v3 support.

I'd like to take this PR a step further and reduce code duplication, but I want feedback first about whether you'd accept this solution before I start refactoring it.

In addition, this change is not backwards-compatible, since it appends the role to your whenever_identifier in the crontab, meaning it won't overwrite the old cron schedule but will instead duplicate it.  I can address this by adding a line of code to the :update_crontab task that removes it from the crontab if it's there.  However, this is risky, since it doesn't make rolling back easy.  An alternative would be to add an installation hook to the gemspec warning of the breaking change.

Please share your thoughts and suggtestions before I proceed.