use env default python in merge_spark_pr.py

A minor change to use env default python instead of fixed `/usr/bin/python`.

Author: Xiangrui Meng <meng@databricks.com>

Closes #965 from mengxr/merge-pr-python and squashes the following commits:

1ae0013 [Xiangrui Meng] use env default python in merge_spark_pr.py

dev/merge_spark_pr.py

-#!/usr/bin/python
+#!/usr/bin/env python
 
 #
 # Licensed to the Apache Software Foundation (ASF) under one or more