paper-icon-theme: no upstream release for a couple of years

[sersorrel]

paper-icon-theme last saw an official upstream release in 2018, but there are some fixes since then which are important for rendering of symbolic icons in e.g. rhythmbox and nemo (see e.g. snwh/paper-icon-theme#739). it would be useful if nixpkgs could use the latest upstream commit rather than the last release.

Checklist

• Checked the nixpkgs master branch
• Checked the nixpkgs pull requests

Project name

nix search name: paper-icon-theme
current version: 2018-06-24
desired version: unstable-2020-03-12

Notify maintainers

maintainers: @romildo

Note for maintainers

Please tag this issue in your PR.