update gnutls packages for Debian

[Quantisan]

Closes #43

Tested on Debian Jessie and working.

[nalimilan]

Ah, sorry, turns out I was wrong about the changes I suggested. WinRPM.RPM supports this syntax, but AptGet and Yum do not. Looks like you need to do something like this:

provides(AptGet, @compat Dict("libgnutls28"=>gnutls, "libgnutls-deb0-28"=>gnutls))

I'll discuss this issue in BinDeps at JuliaPackaging/BinDeps.jl#147.

[Quantisan]

It's weird that my install worked if this isn't supported yet. Perhaps I'm trying it wrong?

I started a fresh Docker container using julia:0.3 image, which uses Debian Jessie. Then I run julia and do a Pkg.clone(<link to my forked github>). It installed fine for some reason.

[nalimilan]

Well, no idea, but it fails the Travis checks, and it didn't work locally here on 0.4... Maybe gnutls was already on the image?

[staticfloat]

The proper way to do this is like this:

provides(AptGet,"libgnutls28",gnutls,validate = pkgmanager_validate)
provides(AptGet,"libgnutls-deb0-28",gnutls,validate = pkgmanager_validate)

Note that you would also need to add in a new library name alias, I submitted a PR to the same effect just now, I didn't see this one before. Feel free to incorporate my changes into this PR; if you do so, I'll close mine.