Cleanup Bower/NPM Packages

- Remove nunjucks from bower; we’re only using it on the server and are
already installing it there through npm.

- Remove the bower-require package that NPM was managing (we weren’t
using any of its features) and simply replace it with (an updated
version of) the standard requirejs bower package
Update flight, skrollr
Use my skrollr-menu fork

Esp, specify it in in bower.json so we don’t accidentally overwrite the
installed files with Prinzhorn’s
version
Simplify requirejs config

Skrollr works with amd, so there’s no need to check for a global export