generator: Use search $PATH for python

* Search $PATH for the python binary so that this works better with
  things like virtualenv as used on Travis CI

diff --git a/generator/nanopb_generator.py b/generator/nanopb_generator.py
index df97338..82b7927 100755 (executable)
--- a/generator/nanopb_generator.py
+++ b/generator/nanopb_generator.py
@@ -1,4 +1,4 @@
-#!/usr/bin/python
+#!/usr/bin/env python
 
 from __future__ import unicode_literals