Upgrade to newer package versions

I went ahead and bumped some package versions in a project I was working on and this is the result. This gives the same simple todo app structure without any deprecation warnings. I didn't manage to update `gulp-useref` however without breaking the entire build process, dunno quite what is up with that.

Also as gulp-clean said to use gulp-rimraf, which in turn said to see the `del` example in the Gulp docs, I went with using `del`.