Ensure MonoGame builds successfully on Linux

On Linux, MonoGame doesn't currently build successfully because the app.config file has different capitalization on disk than it does in the definition file.  

Without this change, you get:

```
/usr/lib/mono/4.5/Microsoft.Common.targets: error : Cannot copy /home/james/Projects/Redpoint/MonoGame/Tools/Pipeline/app.config to /home/james/Projects/Redpoint/MonoGame/Tools/Pipeline/bin/Linux/AnyCPU/Debug/Pipeline.exe.config, as the source file doesn't exist.
```

(because it's actually `App.config` on disk)