build_tools/precommit_checker.py: don't hard-code a platform-afflicted python path (#6124)

Summary:
Use `#!/usr/bin/env python2.7` instead.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/6124

Test Plan: `J=8 make commit_prereq`

Differential Revision: D18834668

Pulled By: ltamasi

fbshipit-source-id: cec40266cd5bcae8bf6cbe5a564ae78540deccc4

build_tools/precommit_checker.py

@@ -1,4 +1,4 @@
-#!/usr/local/fbcode/gcc-4.9-glibc-2.20-fb/bin/python2.7
+#!/usr/bin/env python2.7
 # Copyright (c) Facebook, Inc. and its affiliates. All Rights Reserved.
 
 from __future__ import absolute_import