Wrong version of GSL (#3359)

Based on the file environment.yml in this repository, the GSL version used is 2.2.1. Yet CMakeLists.txt insists on version 19. That is wrong. The latest stable release of GSL is 2.5. Maybe there 1.9 was meant. In either case, apparently this version number is ignored anyway, the only reason I found out is when I switched from your homegrown isis/cmake/FindGSL.cmake when does not use the version in its logic to miniconda3/envs/isis3/share/cmake-3.11/Modules/FindGSL.cmake which does.