Use require whenever available to load viz.js

[colehaus]

It used to be the case that I'd get an error like
when trying to use this library in the browser. The offending code is here:
.

The problem is that the existing code expects browsers to always have loaded viz.js as a globally available script and that require is never available in browsers. Using pulp's browserify command makes both assumptions incorrect (as you can see in the second screenshot, browserify makes a require function available).

Now the code tries require first and falls back to a global Viz only if require fails.

I tested the new code on the command line (via pulp test), in the browser (via pulp browserify) with require, and in the browser without require (manually edited the JS bundle to skip the require attempt).

[colehaus]

Thanks!