Add Ubuntu 18.04 lib

[bording]

As identified in dotnet/roslyn#29289, we need a separate library for Ubuntu 18.04 because of a change to the version of curl it ships.

[bording]

It appears that the AppVeyor build is queued, but has never run.

[gep13]

Just wanted to touch base to see if this is something that can be pulled in? Thanks!

@ethomson @bording

[ethomson]

Oy. I missed the notification for this. I guess we can pull this in, but holy cow do I want to rid ourselves of curl as a dependency as soon as humanly possible.

@gep13 You may also wish to check out libgit2/libgit2sharp#1618 which massively reduces the native dependencies. If you don't need networking, it would be nice to have the feedback on how it's working for you.

[gep13]

@ethomson thanks for the heads up, I will speak to the guys to see what they think.

[gep13]

@bording are you in a position to get this PR updated with the latest changes?

[bording]

I won’t be able to resolve the merge conflicts until sometime tomorrow, but I’ll do that and we can get something out that has this included to at least unblock people until we can get the real fix in place!

[bording]

I'm going to go ahead and merge this in, since it will unblock people until we have the real fix in place.

[bording]

FYI, while testing the new lib, I've noticed something in the ldd output from an 18.04 VM that has me thinking I need to tweak something else here first, so I'm holding off on releasing the new nativebinaries package for now.

[bording]

Nevermind, false alarm. I was accidentally running the wrong distro when I was doing my testing. When I actually used 18.04, everything looks fine.

Before pushing out a new version of the package, I'd like to finish testing on #79 and include that fix as well.