Make the REPL use the global context to be consistent with the node REPL.

This will make packages that modify native prototypes - e.g. [`colors`](https://github.com/Marak/colors.js), [`sugar`](https://github.com/andrewplummer/Sugar) - work as expected.

To verify that the `node` REPL uses the global context, execute `global
=== module.exports.repl.context`.

Note: Tests pass, except `cluster.coffee`, which, however, failed even
before these modifications.