Updating request dependency

Fixes an issue with a vulnerability in the **qs** module.
See https://nodesecurity.io/advisories/qs_dos_extended_event_loop_blocking and https://nodesecurity.io/advisories/qs_dos_memory_exhaustion.

---

```sh
$ git clone https://github.com/mhkeller/alphabits.git alphabits
$ npm install

$ npm shrinkwrap --dev
wrote npm-shrinkwrap.json

# $ sudo npm i nsp -g
$ nsp audit-shrinkwrap --no-color
Name  Installed  Patched  Vulnerable Dependency
qs      0.6.6     >= 1.x  alphabits > request
qs      0.6.6     >= 1.x  alphabits > request

$ npm outdated --depth 0
Package  Current  Wanted  Latest  Location
request   2.36.0  2.36.0  2.40.0  request
```