kernel: take gophernotes from $PATH
kernel: take gophernotes from $PATH

This CL simplifies and harmonizes the way 'gophernotes' is started
from inside Jupyter.
Just take it from $PATH instead of hard-coding some absolute path.

muicoder/gophernotes,gophergala2016/gophernotes,gopherdata/gophernotes,muicoder/gophernotes,muicoder/gophernotes