Use gcc-4.9-glibc-2.20-fb python in precommit_checker

Summary:
The gcc-4.8.1-glibc-2.17 platform is deprecated and will be removed
soon.
Closes https://github.com/facebook/rocksdb/pull/1839

Differential Revision: D4509684

Pulled By: siying

fbshipit-source-id: 3efe296

build_tools/precommit_checker.py

-#!/usr/local/fbcode/gcc-4.8.1-glibc-2.17-fb/bin/python2.7
+#!/usr/local/fbcode/gcc-4.9-glibc-2.20-fb/bin/python2.7
 
 from __future__ import absolute_import
 from __future__ import division