Revert "Disable assembly signing, .NET 4.5.1 doesn't like the keyfile"

This reverts commit 55f1f48ab46fe8728beb0bb58118689d26a6452c.
Our test assemblies are signed and so need gui-unit signed in order
to build.

Whatever the issue with .NET 4.5.1 is, we'll have to solve it a
differnet way.