Update all dependencies to their latest versions.

Remove `grunt-cli` from the dependencies since the proper way to install it is globally with `npm install -g`.
Fix `npm run build`.

`grunt` is in PATH so there's no need to use the relative path. Fixes build on Windows.
Remove trailing whitespaces.
Enable `report: 'min'` for cssmin and uglify tasks.
Add an `npm test` script.

Maps to `grunt dev`.
Use the shorthand `border` property when possible.
README.md: Add devDependencies status badge.