Use OS python version for test suite (#71)

Changes /usr/bin/env python2 to /usr/bin/env python now that the scripts
are Python 3 compatible.

gtest_parallel_tests.py

diff --git a/gtest_parallel_tests.py b/gtest_parallel_tests.py
index 2f227ee..a8ff350 100755
--- a/gtest_parallel_tests.py
+++ b/gtest_parallel_tests.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2
+#!/usr/bin/env python
 # Copyright 2017 Google Inc. All rights reserved.
 #
 # Licensed under the Apache License, Version 2.0 (the "License");