This commit also fixed a broken cmake dependency with unicorn

Ubuntu 18.04 already has cmake 3.10.2

Adds a cmake custom target that will build unicorn on first compile and
uses this in the build scripts as well. Updates Appveyor and Travis
build scripts to work with the new unicorn build, and updates the paths
to all of the different artifacts.

This commit also separates clang-format from the linux build, closing #2702.