allowing setting the bundle executable like d8cfb40f9

I noticed that we are using an old version of capistrano-unicorn. I was trying to use the unicorn_bundle variable to start unicorn with a ENV VAR, but that didn't exist in our current setup. (also I wanted to check the settings with unicorn:show_vars)

This patch will allow me to set the ENV VAR as needed, but I'd like us to move to a newer version of capistrano-unicorn if possible. I'm not sure what changes we've made to our fork.

As far as the surface level it doesn't look like we'd have to make much changes to get to the newest version of capistrano-unicorn (the main thing I saw was we'd have to populate the unicorn role and then make sure the appropriate unicorn hook is called after deploy:restart)