HEMCO submod update: Merge PR #266 (Use short names in gitmodules)

This commit informs the GCHP superproject about the following
commits that were pushed to the GitHub geoschem/HEMCO repository:

3232ba9 Merge PR #266 (Now use short submodule names in .gitmodules)

Signed-off-by: Bob Yantosca <yantosca@seas.harvard.edu>