Update Piggieback to 0.1.2 and remove explicit setup step from run-node-nrepl

I was getting NullPointerExceptions while trying to use cljs-noderepl with nREPL/Piggieback as documented. 

After updating to the latest Piggieback, I realised that the call to `node-setup` in `nrepl-env` was causing problems and also unnecessary since version 0.1.0.

Removing it solves the problem. The REPL can now be launched using the 3 prescribed methods in the README.