gem sources --list doesn't print anything to the terminal

Commit d58c5b2abda70c6819950e0174d61f05e1a3d3d1 introduces a bug in the "gem sources" command.

When "--list" or "-l" is passed to "gem sources", the command is currently ignoring it (nothing is printed to terminal).

Currently, when options[:list] is set (by passing -l or --list), the list() method is never reached.