Update .gitmodules URLs to use HTTPS instead of SSH

[rawrmaan]

I ran into the following issue when deploying to Heroku with this package in the package.json:

-----> Installing dependencies
       Installing node modules (package.json + package-lock)
       Unhandled rejection Error: Command failed: /usr/bin/git submodule update -q --init --recursive
       warning: templates not found /tmp/pacote-git-template-tmp/git-clone-1f26257e
       Host key verification failed.
       fatal: Could not read from remote repository.
       
       Please make sure you have the correct access rights
       and the repository exists.
       fatal: clone of '[email protected]:adjust/android_sdk.git' into submodule path 'ext/android/sdk' failed

This article explains why you should always declare git submodule URLs as HTTPS rather than the [email protected]: SSH form for maximum compatibility. This PR makes that simple change, fixing the issue.

[uerceg]

Replaced with #70.