Prevent "meteor shell" from stomping Underscore

The return value of last REPL command is stored in global._ if the REPL is set to use the global context. So after you run your first REPL command, Underscore is no longer available to Meteor server code as global._ - that variable is now the result of your last REPL command.

See http://stackoverflow.com/questions/10973968/underscore-doesnt-work-in-coffeescripts-console