Fix #2971 finally!

The accepted answer on
https://stackoverflow.com/questions/14695845/why-does-grunt-fail
meant to install Grunt globally, and it didn't specify that
`npm run-script grunt` is not automagical, but uses `scripts`
section of Gruntfile.js, which is absent in our project.

Makefile

@@ -67,7 +67,7 @@ node_modules: package.json
 	@if [ -d node_modules ]; then touch node_modules; fi
 
 jstest: node_modules
-	npm run-script grunt-cli test
+	node_modules/grunt-cli/bin/grunt test
 
 transifexrc:
 	@echo '[https://www.transifex.com]' >.transifexrc