Track submodule heads.

`git submodule update --remote` will now sync the submodules
to their master branch.

Change-Id: I192ee8153d9538a599ce63fd5d51ade9741bc6fd
Reviewed-on: http://gerrit.dmdirc.com/3597
Reviewed-by: Greg Holmes <greg@dmdirc.com>
Automatic-Compile: DMDirc Build Manager