Invoke the koan-engine ns initializer.

The idea behind this is to clean-up the repl initialization as proposed
in pull request #68 such that it's taken care of in a back compatible
way from the koans-engine project.

This pull request is the alternative to #71.  It's implementation will require
functional-koans/clojure-koan-engine#10 to be merged

More details of the pros and cons of this approach are provided in that pull
request as the majority of the work is handled by the koan-eninge project.