history: a bit more explicit
Notice changes to npm-shrinkwrap.json

There were two separate issues here:

 - npm-shrinkwrap.json wasn't actually being used as part of the
   watchset/buildinfo, so changes to it might not cause the package to
   be considered for rebuilding

 - meteor-npm only compared top-level changes when deciding whether to
   update, not all changes

Fixes #1648
update sockjs to 0.3.9, websocket-driver to 0.3.4

The websocket-driver update includes our PR to relinquish some memory
faster. https://github.com/faye/websocket-driver-node/pull/6

The sockjs update mostly consists of aligning the version of
faye-websocket it uses internally with the version that we use for our
client. It also contains this Vary: Origin change:
 https://github.com/sockjs/sockjs-node/pull/130