Update development settings to use BOXEN_SOCKET_DIR

By default the Boxen install doesn't have the `GH_SOCKET_DIR` env variable set, but it does have `BOXEN_SOCKET_DIR` available and present. This causes ./script/server to fail on launch with the following error as the env variable is `nil`:

    config/unicorn/development.rb:1:in `reload': undefined method `+' for nil:NilClass (NoMethodError)

This updates the *development* settings to use the Boxen socket dir fixing the problem.