call correct highlightNodes function, fixes #550

colourHighlightNodes is never added to the global object, so calling it that
way resulted in a "function undefined" error. I believe this was due to an
earlier PR comment where I recommended we not add it to global because it is
only ever used inside graph.js. This is probably a leftover line that wasn't
changed for that comment.
Merge pull request #559 from mozilla/fix/empty-graph-after-blocking

call correct highlightNodes function, fixes #550