Fix bug

I use knockout-es5-passy in Google Chrome as extension. Thanks. By the way, I found some error.

```
Refused to evaluate a string as JavaScript because 'unsafe-eval' is not an allowed source of script in the following Content Security Policy directive: "script-src 'self' chrome-extension-resource:".

Uncaught EvalError: Refused to evaluate a string as JavaScript because 'unsafe-eval' is not an allowed source of script in the following Content Security Policy directive: "script-src 'self' chrome-extension-resource:".
```

In Google Chrome environment, `eval` is forbidden. Any help would be appreciated.