nbconvert: Make sure node is atleast version 0.9.12

Trying to export html using nbconvert and an older version of node fails silently.  All of the markdown gets stripped from the document.  I used `nvm` to install and test `node.js` with our `marked.js` for node versions 0.6+.  I discovered only node versions 0.9.12 and up produce any output. 0.8 and earlier produce no output and 0.9.11 and earlier hang.

This PR adds a test that makes sure that node is the right version before deciding to use it, otherwise pandoc is used.