add a shebang line to all Ruby examples + chmod to be executable

2 of the 47 Ruby examples have shebang lines and are marked as executable in the repo.  It makes sense to leave the shebang line out when typesetting an inline code example but it's also very convenient to be able to directly execute the examples (assuming a POSIX like shell) without typing the interpreter name.  A slightly less trivial alternative to this patch would be to remove the shebang lines from the few examples that have one and include a trivial script to insert a shebang + chmod the examples.