Should gracefully handle missing dependencies. Fixes substack/factor-bundle#33

If we can't look up a dependency, we should assume it's been exported
via some other means, rather than causing an exception.

This could be caused by Browserify's 'exclude' or if using `require`
with an exposed name.
Merge pull request #1 from terinjokes/missing-deps

Should gracefully handle missing dependencies. Fixes substack/factor-bundle#33