Upgrade should to avoid performance problems in node 7.x.

Node 7.x contains a performance regression which makes certain
`instanceof` tests 100x slower:
  nodejs/node#9634

Upgrading `should` from 8.3.0 to 11.1.2 makes this performance
problem go away when running our tests, for reasons I don't
fully understand, but it seems to be related to:
  shouldjs/should.js@0737171
so perhaps the root of the performance problem is actually
`should.not`, not the `instanceof` regression.

package.json

@@ -20,6 +20,6 @@
   "devDependencies": {
     "jshint": "^2.9.1",
     "mocha": "^2.4.5",
-    "should": "^8.3.0"
+    "should": "^11.1.2"
   }
 }