Use https url for submodule

If we use the ssh transport, the user must have a vaild ssh key
configured for github (irrespective of write access to the repo).

Instead use https, which does not require login and should be the most
commonly available transport.

Signed-off-by: Joel Stanley <joel@jms.id.au>
Merge pull request #8 from shenki/https-submodule

Use https url for submodule