updated nunjucks version to 2.4.3 (#603)

This seemed stalled on @Blackweda, so I went ahead and did it.  I've stuck with `^2.4.3` even though master has it at `3.0.0`.  If we want to update to `3.0.0` we can do that in another patch.

package.json

@@ -52,7 +52,7 @@
         "gunzip-maybe": "^1.3.1",
         "http-server": "^0.9.0",
         "mkdirp": "0.5.1",
-        "nunjucks": "^3.0.0",
+        "nunjucks": "^2.4.3",
         "properties-parser": "0.3.1",
         "request": "^2.69.0",
         "requirejs": "^2.3.3",