Make submodule url more consistent with other url's

also fixes the following issue:

Submodule 'm4' (git://github.com/savonet/m4.git) registered for path 'm4'
git submodule update --recursive
Cloning into m4...
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
Clone of 'git@github.com:savonet/m4.git' into submodule path 'm4' failed
Failed to recurse into submodule path 'liquidsoap'
make: *** [init] Error 1