kiji-express: EXP-255. Run method in the REPL should reset the REPL's flowDef.

issue: https://jira.kiji.org/browse/EXP-255
review: https://review.kiji.org/r/1277
commit: https://github.com/kijiproject/kiji-express/commit/c01fa32aaf0fa28483fcd0a060c6d4615697c98d