Upgrade driver: mongodb to 3.1.3 and mongodb-core to 3.1.2, fixes #6692 & #6670

[simllll]

upgrades underyling mongodb driver:

• fixes mongos reconnect issues
• a lot of other features/bug fixes in mongodb driver, see: https://github.com/mongodb/node-mongodb-native/blob/master/HISTORY.md

[simllll]

Not sure what's happening with the travis build,

1) transactions
       "before all" hook:
     Uncaught TypeError: Cannot read property 'close' of undefined
      at topology.connect (node_modules/mongodb/lib/operations/mongo_client_ops.js:412:16)
      at ReplSet.<anonymous> (node_modules/mongodb/lib/topologies/replset.js:364:11)
      at node_modules/mongodb-core/lib/topologies/replset.js:634:14
      at Server.<anonymous> (node_modules/mongodb-core/lib/topologies/replset.js:357:9)
      at node_modules/mongodb-core/lib/topologies/server.js:508:16
      at node_modules/mongodb-core/lib/connection/pool.js:531:18
      at process._tickCallback (internal/process/next_tick.js:61:11)

seems like an underlying library issue, I've created a new PR there mongodb/node-mongodb-native#1812 to fix this

[vkarpov15]

Thanks for your patience and digging into this issue, but unfortunately I can't merge this until mongodb/node-mongodb-native#1812 is merged.

[vkarpov15]

d907dc2 is enough to make tests pass locally for me, so I'll merge this 👍