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.

Author: sbinet

kernel/kernel.json

@@ -1,9 +1,9 @@
 {
     "argv": [
-    	"/go/bin/gophernotes", 
+		"gophernotes",
     	"{connection_file}"
     	],
     "display_name": "Golang",
     "language": "go",
     "name": "go"
-}
+}