[plugin-observable] Observables not working as expected

Hi again :)

Observables aren't working as expected. In particular, I've a TC39 implementation of Observables which is not being interpreted correctly.

The problem would seem to be the `isObservable` function, which adds this to `jss`:

```js
var symbolObservable = lib;

var isObservable = (function (value) {
return value && value[symbolObservable] && value === value[symbolObservable]();
});
```

Here's some command line output that's illuminating:

```js
>>> Symbol.observable === symbolObservable
false
>>> symbolObservable.default === Symbol.observable
true
```

It's possible there's some weird things going on with the build process on my end (perhaps I need some webpack-commonjs bridge), but I didn't see any tests in jss that demonstrate the use of `Symbol.observable` proper, so I can't easily eliminate the possibility that it's a jss issue.