Update for recent CodeMirror change for Sprint 36

This is for #163 

This fix uses some educated guesses derived by looking at [this commit](https://github.com/adobe/brackets/commit/76a5e7de8a6e43b894187a46c71ef1f9d1fc3c36#diff-205888d21d20aa2319d86062c2e86080) by @njx 

cc @RaymondLim