Since `mz` has already bumped for several times without
breaking change, it's better to follow the updates.
```console
$ npm outdated
Package Current Wanted Latest Location
mz 2.1.0 2.1.0 2.7.0 fission-nodejs-runtime
```
Fixes: https://github.com/fission/fission/issues/167
Upgrades the node environment to NodeJS 7.6.0. Functions can now use async/await and promises; they can return a promise instead of using a callback. The change preserves compatibility with the callback style.
* Upgrade node environment to Node.js 7.6.0+
* Bump package version
* Preserve compatibility for callbacks
* Cleanup
* Re-add hello callback example
* Make sure not returning won't blow everything up
* Update stock example with request promise
