C#: change keyserver to keyserver.ubuntu.com

The pgp.mit.edu server seems pretty unreliable lately, e.g. it failed on [this](https://travis-ci.org/gnome-do/core/builds/44062038#L84) and [this](https://travis-ci.org/akoeplinger/omnisharp-roslyn/builds/44084881#L84) build.

I've heard that the Ubuntu keyserver should be more reliable.