Number.isInteger in IE11

I'm having this error when importing and using `readable-stream` in IE11
```
TypeError: Object doesn't support property or method 'isInteger'
       at getHighWaterMark (eval code:12:5)
```

It seems like `isInteger` in only used in `/lib/internal/streams/state.js` and can be fixed during build step

Here is a PR #389 fixing it