chore(docs): use Docker Toolbox, not boot2docker

Issue #4266 notes how boot2docker is now deprecated. This commit
changes documentation and Makefile notices to recommend using
Docker Toolbox and Docker Machine instead.

Author: technosophos

Makefile

@@ -56,7 +56,7 @@ postgres:
 	docker start postgres || docker run --restart="always" -d -p 5432:5432 --name postgres postgres:9.4.1
 	docker exec postgres createdb -U postgres deis 2>/dev/null || true
 	@echo "To use postgres for local development:"
-	@echo "    export PGHOST=`boot2docker ip 2>/dev/null || echo 127.0.0.1`"
+	@echo "    export PGHOST=`docker-machine ip $$(docker-machine active) 2>/dev/null || echo 127.0.0.1`"
 	@echo "    export PGPORT=5432"
 	@echo "    export PGUSER=postgres"