Needless dependency on globally installed grunt-cli

[dancek]

Travis CI doesn't pre-install grunt-cli in current versions of Node.js, which makes npm test fail for the current test script (grunt test).

It's very easy to avoid the problem by adding grunt-cli as a dev dependency and making npm test call Grunt using npm run, which has node_modules/.bin in $PATH. For example like I did here: https://github.com/dancek/grunt-more-css/commit/dba41c9ab5bf051f28dc8f8c28257ecb457cc5be

The related Travis CI bug: https://github.com/travis-ci/travis-ci/issues/3231