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

Node 7.x contains a performance regression which makes certain
`instanceof` tests 100x slower:
  https://github.com/nodejs/node/issues/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:
  https://github.com/shouldjs/should.js/commit/073717122cbe1275d6b8346c2d25b78c353ef333
so perhaps the root of the performance problem is actually
`should.not`, not the `instanceof` regression.

fgnass/domino,cscott/domino,arlolra/domino,arlolra/domino,fgnass/domino,cscott/domino