Don't use variable inside of require in index.js

Backstory: I tried bundling code which uses underscore-contrib for browsers with browserify. It complained about not being able to find 'underscore.array.builders'. The reason lies in the way modules are being required in contrib's `index.js`: `require('underscore.'+lib)`. browserify only recognizes constant values passed to `require`, e.g., `require('underscore.array.builders')`.

Stylistically I prefer the current solution but I feel it's more important to have statically named dependencies so compilers & bundlers can recognise and work with them.