Make .NET 4.5 the default for development - I hope this time for real.

DotPeek is showing the right framework versions:

![image](https://cloud.githubusercontent.com/assets/57396/3385917/3cddb3ae-fc6f-11e3-946f-38d8da1dc3ed.png)

and FAKE is not complaining any more. Hope it's finally fixed. Sorry