Switch from using huxley to node-huxley

Huxley had some strange problems where screenshots kept getting messed up by
becoming different sizes than they were before. Node-huxley seems to have fixed
this problem, so we'll try using that instead.

Also, fix the sizing tests to the new syntax.

Auditors: alpert

test/huxley/NestedFractions.hux/record.json

@@ -0,0 +1,5 @@
+[
+  {
+    "action": "screenshot"
+  }
+]