require requirejs (fixes #52)

Note that I have no idea what I'm doing; I know nothing about requirejs.
I saw this post:
https://stackoverflow.com/questions/14548369/requirejs-relative-paths#comment20294321_14548369
and I saw that `requirejs` is undefined. So let's define it.

Fixes #52.
Merge pull request #99 from tjanson/fix-52

require requirejs (fixes #52)