Made it possible to use fallbacks for CDN paths

With current behaviour, when arrays just removed from build_config it is impossible to use fallbacks paths, because r.js still looking for dependency module and precompilation fails with error "Error: ENOENT, no such file or directory ..."

With this changes configuration like this
config/requirejs.yml:
```yaml
paths:
  jquery:
  - http://cdn1.com/jquery-1.9.1.min
  - http://cdn2.com/jquery-1.9.1.min
  - jquery-1.9.1
```

works as follows: replace paths array with "empty:" in build_config for such module
and adds this paths directly into run_config on page.
Make sure local fallback is precompiled on your server.