Do not use hardcoded bash

Index: compiledb/commands/make.py
--- compiledb/commands/make.py.orig
+++ compiledb/commands/make.py
@@ -23,7 +23,7 @@ class AutoconfMockScript:
     def __init__(self, verbose):
         self.verbose = verbose
         self.path = None
-        self.mock_script = """#!/bin/bash
+        self.mock_script = """#!/usr/bin/env bash
             ## Auto generated by compiledb. Do not modify!
             ## https://github.com/nickdiego/compiledb
             case "$*" in
