Fix for 'TopoJSON library not loading in IPython notebook' in Issue #93

This fixes https://github.com/wrobstory/vincent/issues/93

I used the same pattern that was used to load d3.js. I noticed the recent issue where d3 was failing to load and used the same pattern in vincent/core.py: https://github.com/wrobstory/vincent/pull/90 and I think the same