update .gitmodules use public url to prevent permission denied on recursive checkout

For solving the following error on recursive checkout
Submodule 'libs/zsLib' (git@github.com:openpeer/zsLib.git) registered for path 'libs/zsLib'
Cloning into 'libs/boost'...
Permission denied (publickey).
fatal: Could not read from remote repository.

Please make sure you have the correct access rights
Merge pull request #2 from evermeer/patch-1

update .gitmodules use public url to prevent permission denied on recurs...