Ensure that npm dependencies are installed if package.json is modified

With the following steps, the second invocation of `make` should trigger
installation of npm dependencies:

 1. Run `make`
 2. Edit dependencies in `package.json`
 3. Run `make` again

Makefile

@@ -24,6 +24,6 @@ lint: node_modules/.uptodate
 build/manifest.json: node_modules/.uptodate
 	npm run-script build
 
-node_modules/.uptodate:
+node_modules/.uptodate: package.json npm-shrinkwrap.json
 	npm run-script deps 2>/dev/null || npm install
 	@touch $@