lookup: use GitHub head for serialport (#992)

This will hopefully fix serialport when run against a `-pre`
version of Node.js. Eventually, this fix will be in a release
of serialport, but it's in GitHub now.

Closes: #991

Author: Trott

lib/lookup.json

@@ -420,7 +420,7 @@
     "comment": "postlint script fails"
   },
   "serialport": {
-    "prefix": "serialport@",
+    "head": true,
     "tags": "native",
     "maintainers": "reconbot"
   },