Use https address instead of ssh:// address for libevent submodule.

If you use the ssh:// address people that are not part of the collaborators for
that repo will not be able to do a submodule init, but will get a public key
permission denied error.
Merge branch 'fix/submodules' into foo

* fix/submodules:
  Use https address instead of ssh:// address for libevent submodule.