Revert "Add 5.20 to Travis CI builds" as it looks like Travis has issues with 5.20
Revert "Add 5.20 to Travis CI builds" as it looks like Travis has issues with 5.20

This reverts commit 4522b7f31b7e753f4e19af60e55633d7c3e0ee52.

lordofhyphens/Slic3r,curieos/Slic3r,prusa3d/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,prusa3d/Slic3r,mcilhargey/Slic3r,pieis2pi/Slic3r,prusa3d/Slic3r,curieos/Slic3r,alexrj/Slic3r,mcilhargey/Slic3r,alexrj/Slic3r,platsch/Slic3r,prusa3d/Slic3r,pieis2pi/Slic3r,lordofhyphens/Slic3r,tmotl/Slic3r,prusa3d/Slic3r,tmotl/Slic3r,pieis2pi/Slic3r,platsch/Slic3r,lordofhyphens/Slic3r,xoan/Slic3r,mcilhargey/Slic3r,xoan/Slic3r,lordofhyphens/Slic3r,tmotl/Slic3r,lordofhyphens/Slic3r,tmotl/Slic3r,curieos/Slic3r,xoan/Slic3r,platsch/Slic3r,alexrj/Slic3r,alexrj/Slic3r,xoan/Slic3r,mcilhargey/Slic3r,mcilhargey/Slic3r,tmotl/Slic3r,xoan/Slic3r,tmotl/Slic3r,lordofhyphens/Slic3r,alexrj/Slic3r,pieis2pi/Slic3r,curieos/Slic3r,pieis2pi/Slic3r,mcilhargey/Slic3r,prusa3d/Slic3r,alexrj/Slic3r,platsch/Slic3r,platsch/Slic3r,platsch/Slic3r,xoan/Slic3r,mcilhargey/Slic3r,curieos/Slic3r,prusa3d/Slic3r