Avoid creating unnecessary production build when running `make dev`

`make dev` would unnecessarily create a production build before starting
the development server if `package.json` or `yarn.lock` had been touched
since the previous run of `make dev`. This was happening because `make dev`
had a dependency on `build/manifest.json`, which triggers a production
build, rather than `node_modules/.uptodate`, which just runs `yarn
install`. I can't remember the historical reason why it was this way, but it
doesn't make sense any more.

Fix this by changing `make dev` to only depend on `node_modules/` being
up to date before starting the dev server.

Makefile

@@ -17,7 +17,7 @@ help:
 	@echo "                       dependencies, etc)"
 
 .PHONY: dev
-dev: build/manifest.json
+dev: node_modules/.uptodate
 	node_modules/.bin/gulp watch
 
 .PHONY: test