Public repository url for the libjson-rpc submodule

The url of the submodule should be the https one,
because the current one doesn't allow to clone it,
if you don't have an account on GitHub
(or no configured access on your computer).
Merge pull request #9 from MiLk/fixes/public-submodule

Public repository url for the libjson-rpc submodule
Merge branch 'develop' into features/cmake