Use mongodb-version-manager for matrix tests

Want to test against 2.6.6? `MONGODB_VERSION=2.6.6 npm test;`

Want to test against latest 2.8 rc? `MONGODB_VERSION=2.8.x npm test;`

2.2.x functional tests are currently broken through:

```
> MONGODB_VERSION=2.2.x npm test;
# …lots of logging output
error command line: unknown option setParameter
use --help for help
```

Perhaps we should collab more on things like
https://github.com/imlucas/mongodb-runner and
https://github.com/imlucas/mongodb-bridge :)