Update .gitmodules

The changes above should fix the following clone error:

git submodule update --recursive
Cloning into core...
Permission denied (publickey).
fatal: The remote end hung up unexpectedly
Merge pull request #6 from dcerisano/patch-1

Update .gitmodules
Merge branch 'master' of github.com:h5p/h5p-wordpress-plugin into user-results
Implemented missing interface functions.