Fix `npm test` command path

[sanemat]

This requires global grunt-cli. We have two solution: A: npm install -g grunt-cli before (e.g. travis-ci: before_script section) B: Change npm test script path and use script/test

I choose the latter, because script/test takes account of environment variables I think.

[desmondmorris]

+1 @sanemat. Its good you chose B because there is a fix pending merge that addresses A (#26).