Makefile: explicitly create bin/, if necessary

Most of the go(1) tools will create the missing directory.
Yet, it is better to define this dependency explicitly to make(1),
to avoid unnecessary surprises.

Another useful feature of make(1) is the multiple targets per rule.
This way, we can have the same action and it is the target the variable
of the rule, anymore.

Makefile

@@ -22,8 +22,7 @@ test:
 	go test ./...
 	go vet ./...
 
-release: bindata.go
-	@echo mkdir bin 2>/dev/null || true
+release: bin bindata.go
 	go build -ldflags "-X main.Version=$(VERSION)" -o bin/pomo-$(VERSION)-linux
 
 docs: readme
@@ -32,5 +31,5 @@ docs: readme
 readme: www/data
 	cat README.md | python -c 'import json,sys; print(json.dumps({"content": sys.stdin.read()}))' > www/data/readme.json
 
-www/data:
+www/data bin:
 	mkdir -p $@