Point submodule remotes to https address

Previously the two git submodules ([json11](https://github.com/dropbox/json11) & [CppSQLite](https://github.com/neosmart/CppSQLite)) were pointing to GitHub repos via SSH (git@github.com…), but accessing these repos requires publickey access (i.e., repo membership). Updating submodules would result in the following error:

    Cloning into 'deps/CppSQLite'...
    Permission denied (publickey).
    fatal: Could not read from remote repository.

    Please make sure you have the correct access rights
    and the repository exists.
    Clone of 'git@github.com:neosmart/CppSQLite.git' into submodule path 'deps/CppSQLite' failed

Using GitHub's https protocol fixes the problem.

Once these changes are pulled in, make sure you update your working copy:

    git submodule sync

Thanks! I’m very excited about [mx3](https://github.com/skabbes/mx3)!