Output on stderr is always considered an error

The sass compiler can output messages like:

```
DEPRECATION WARNING:
Sass 3.5 will no longer support Ruby 1.9.3.
Please upgrade to Ruby 2.0.0 or greater as soon as possible.
```

This message is not fatal, and in fact sass still returns the status code 0, but since django-static-precompiler considers any output on stderr an error, the compilation fails anyway.
