Include dynamic PLEK_SERVICE_STATIC_URI env
Include dynamic PLEK_SERVICE_STATIC_URI env

When starting the Heroku review apps we need the static URI that
slimmer looks for to be dynamically set to the current Heroku app.

Otherwise Slimmer will not know where to request assets from.

https://github.com/alphagov/slimmer/blob/master/lib/slimmer/app.rb#L27-L
29

alphagov/static,alphagov/static,alphagov/static