Excluded AutoFixture.AutoFoq from Strong-Name signing

Resolves an issue on the CI server related to Foq and Strong-Name signing.

In order to run a successful build on the CI server the `/p:SignAssembly=true` must be removed from the command line parameters to MSBuild.exe (since it is now explicitly defined on each project file).

(Continuation of [#56](https://github.com/AutoFixture/AutoFixture/issues/56#issuecomment-24673680).)