nvidia-prime depends upon gdm instead of gdm3

https://github.com/tseliot/nvidia-prime/pull/4

[Impact]
- Triaged as High for its impact on Ubuntu GNOME.
- Users who install nvidia-prime currently will get lightdm installed, which triggers a nasty debconf prompt asking them to choose whether to use lightdm or gdm3.
- However, there is no explanation given about what are the advantages or disadvantages of each choice. (For instance, GNOME Shell does not integrate as well with lightdm; I'm guessing Unity has a similar issue with gdm3). Users may easily not know which one is installed by default on their system or which is recommended in their situation.

[Test Case]
1. From Ubuntu GNOME 16.04 LTS, install nividia-prime
2. Is lightdm installed? Are you asked to choose which display manager to use?

[Regression Potential]
None, this simply adds gdm3 as a supported alternative dependency.

[Original Report]
Trying to install nvidia-prime on an ubuntu-gnome system tries to pull in a bunch of unity stuff, because it lists as dependencies:

Depends: lightdm (>= 1.9.1) | gdm | kdm | sddm, bbswitch-dkms, pciutils, lsb-release, lsb-base (>= 4.1+Debian11ubuntu7)

It should depend on gdm3 in addition, since gdm is a transitional package.

ProblemType: Bug
DistroRelease: Ubuntu 16.04
Package: nvidia-prime (not installed)
Uname: Linux 4.5.4-040504-lowlatency x86_64
NonfreeKernelModules: zfs zunicode zcommon znvpair zavl
ApportVersion: 2.20.1-0ubuntu2
Architecture: amd64
CurrentDesktop: GNOME
Date: Mon May 16 19:17:25 2016
InstallationDate: Installed on 2016-05-15 (1 days ago)
InstallationMedia: Ubuntu-GNOME 16.04 LTS "Xenial Xerus" - Release amd64 (20160421)
SourcePackage: nvidia-prime
UpgradeStatus: No upgrade log present (probably fresh install)