xbuild does not support /nr: option and treats this as another project.

:frowning:
```
Creating zip file src/FubuRake.Docs/pak-WebContent.zip
    Starting to write contents to new Zip file at src/FubuRake.Docs/pak-WebContent.zip                                                                            
No matching files for Bottle directory data
No matching files for Bottle directory config
/usr/bin/xbuild src/FubuRake.sln   /property:Configuration=Debug /property:TargetFrameworkProfile= /v:m /t:rebuild /nr:False
MSBUILD: error MSBUILD0004: Too many project files specified
rake aborted!
```

This patch

:smile: again