Patched security issue per csteipp

https://bugzilla.wikimedia.org/show_bug.cgi?id=69623#c2

Github:
https://github.com/nyurik/vega/commit/36f188cc5bc0f8cf2de455f38a6163257547cca6

Pull request:
https://github.com/trifacta/vega/pull/217

Change-Id: I7c0ac875df3300029cc91fcd0674fd9e4bf1e315