Change .gitmodules from git:// to https://

This is required to enable test execution behind
a firewall using parameters -DproxyHost=<host> -DproxyPort=<port>.
CliGitAPIImplTest.test_submodule_tags_not_fetched_into_parent fails
also with proxy settings since git:// required port 9418 to be open.
Using https:// solves this issue.