Bump chai to 1.9.2 to satisfy sinon-chai peer dependency.

Error while installing dependencies.

```
$ npm install
...
npm ERR! peerinvalid The package chai does not satisfy its siblings' peerDependencies requirements!
npm ERR! peerinvalid Peer chai-as-promised@4.1.1 wants chai@>= 1.7.0 < 2
npm ERR! peerinvalid Peer sinon-chai@2.6.0 wants chai@>=1.9.2 <2
...
```

https://github.com/domenic/sinon-chai/commit/9d72b166784384e84e1bad0e60eb16bce53668ed#diff-b9cfc7f2cdf78a7f4b91a753d10865a2R27