Prevent REPL from stomping global._ (Underscore library)

The return value of last command is stored in global._ if the REPL is set to use the global context. So Underscore is no longer available to Meteor server code as global._

See http://stackoverflow.com/questions/10973968/underscore-doesnt-work-in-coffeescripts-console
Merge pull request #3227 from graemian/devel

Prevent "meteor shell" from stomping Underscore.