Mirror the Eigen source tarball - redirect causes issues

The bitbucket.org server is using a redirect which is tripping up
CMake's download command on some systems. Mirror the source file and
change the MongoDB URL.

Change-Id: I416262ba6525ccb289f840c77d5df0092f0edbaf
Merge topic 'submodule-updates' into master

6b4da62b Mirror the Eigen source tarball - redirect causes issues