Added missing comma to fix '-v' and '--version' options

A comma was missing in the options parser that caused the '-v' and '--version' options to fail when using Mercenary. The "--version" option would fail with something like:

    $ mercenary_example --version
    mercenary_example: version unknown

And the '-v' version would fail with something like:

    .../gems/mercenary-0.2.0/lib/mercenary/program.rb:45:in `go': missing argument: -v (OptionParser::MissingArgument)

Adding the comma fixes both issues so that both '-v' and '--version' work as expected.