ENH: Use github for CMake HTTPS testing

URL to the Slicer wiki was used for testing. If Slicer wiki breaks down (which is not uncommon) then it causes the build to fail.
Switched to a github URL (https://raw.githubusercontent.com/Slicer/Slicer/master/CMakeLists.txt) as github is more reliable
and if github is down then Slicer cannot be built anyway (as dependencies are downloaded from github).

git-svn-id: http://svn.slicer.org/Slicer4/trunk@25501 3bd1e089-480b-0410-8dfb-8563597acbee