Prevent potential firewall problem with git clone (#3094)

The better solution might well be to do this locally, using either

```
git config --global url."https://".insteadOf git://
```

or

```
git config --global url."https://github.com".insteadOf git://github.com
```

However, we don't live in an ideal world so this commit intends to lower the barrier to entry.

See https://stackoverflow.com/questions/16298986/unable-to-connect-to-github-com-for-cloning for more information.

.gitmodules

@@ -1,6 +1,6 @@
 [submodule "koreader-base"]
 	path = base
-	url = git://github.com/koreader/koreader-base.git
+	url = https://github.com/koreader/koreader-base.git
 [submodule "platform/android/luajit-launcher"]
 	path = platform/android/luajit-launcher
 	url = https://github.com/koreader/android-luajit-launcher.git