Discourage git:// urls

The `git://` protocol is unencrypted and unauthenticated, and allows a man-in-the-middle attacker to substitute malicious code and compromise the user's system.  We should note this fact in documentation and prefer secure `https://` URLs in examples.

This pull request applies these changes to all versions' manuals.

See also bundler/bundler#2958 and bundler/bundler#2960 for the accompanying manpage changes.
Thanks!