Only call symlink_before_migrate once

.env isn't being symlinked because we're calling symlink_before_migrate
twice in deploy_revision. Rather than calling it twice we should be
passing it a hash of things to symlink.
Test that the unicorn config is being symlinked to shared/unicorn.
Merge pull request #50 from opscode-cookbooks/link-env

Symlink shared/.env.production to current/.env.production