[poppler] Moving up our "minimum" Linux base

[Albert Astals Cid]

Right now our minimum Linux base as "guaranteed" by the gitlab CI is Ubuntu 16.04. 

I'd like to move that to Ubuntu 18.04.

As far as i understand no "old distro" is updating the poppler package, the having relatively old requirements is there to make it easier for "everyone" that wants to build poppler to be able to use their current distribution to do so without having to worry that they need something released last week.

But I *think* that "linux" developers should probably be using a distibution that is newer than 2.5 years.

Opinions?

Cheers,
  Albert