Modify shebang on python scripts

Change shebangs to `#!/usr/bin/env python` instead of
`#!/usr/bin/env python2.7`.
diff --git a/tools/run_tests/run_tests_matrix.py b/tools/run_tests/run_tests_matrix.py
index 191228e..b9a9bd5 100755
--- a/tools/run_tests/run_tests_matrix.py
+++ b/tools/run_tests/run_tests_matrix.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2015, Google Inc.
 # All rights reserved.
 #