Convert output toString() before calling replace()

I'm not sure if this is a necessary pull request, but it doesn't hurt submitting it anyway.

When trying to build Knockout on an Ubuntu 13.10 x64 instance I got the following error at the end of the process:

```
Fatal error: Object true has no method 'replace'
```

I fixed the issue by converting the output to a string before calling replace() on it. This fixed my issue, but if anyone has any suggestions so I can build Knockout without this modification, it'd be greatly appreciated.