generator: Don't force python2

Python2 is being phased out of the default python interpreter.  Arch
Linux has moved some time ago and upcoming Debian realeases will follow.

My distro, Arch, doesn't have a python2-protobuf version, but does have
a python3 version.  With a python 2 & 3 compatible generator the exact
interpreter can be ignored and can use the system default as most users
expect.

Update issue #155
diff --git a/generator/protoc-gen-nanopb b/generator/protoc-gen-nanopb
index 6dc468d..358f97c 100755
--- a/generator/protoc-gen-nanopb
+++ b/generator/protoc-gen-nanopb
@@ -10,5 +10,4 @@
 # --plugin= on the command line.
 
 MYPATH=$(dirname "$0")
-PYTHON=$(which python2 || which python)
-exec $PYTHON "$MYPATH/nanopb_generator.py" --protoc-plugin
+exec "$MYPATH/nanopb_generator.py" --protoc-plugin