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_microbenchmark.py b/tools/run_tests/run_microbenchmark.py
index 9471110..57b2636 100755
--- a/tools/run_tests/run_microbenchmark.py
+++ b/tools/run_tests/run_microbenchmark.py
@@ -1,4 +1,4 @@
-#!/usr/bin/env python2.7
+#!/usr/bin/env python
 # Copyright 2017, Google Inc.
 # All rights reserved.
 #