Loading library with version doesn't work

This block from mbostock doesn't work on blockbuilder

http://blockbuilder.org/mbostock/1483226

It throws an error because it can't load the horizon plugin library with a version

```

removing the version number makes it work

(http://blockbuilder.org/john-guerra/7770baa3383ab777f8ac211deeb66fbc)

Reproduce the failure with the linked mbostock block using , then compare it with the working unversioned example. Trace how Blockbuilder loads external libraries and consider the issue done when the versioned horizon plugin loads without an error.

Written by the indexing model from the issue text.