scripts: change shebang

They say use /usr/bin/env bash instead of /bin/bash because some OSs
don't have bash installed or don't have it installed in /bin/bash.

backend/tools/package_backend_framework.bash

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

backend/tools/push_backend_framework_to_s3.bash

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

create_packages.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e
 

fix_style.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script runs clang-format over all files ending in .h, .c, .cpp listed
 # by git in the given directory.

generate_docs.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Run with argument `--skip-checks` to skip checks for clean build and removing install dir.
 

start_px4_sitl.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script spawns the Gazebo PX4 software in the loop (SITL) simulation.
 # Options:

stop_px4_sitl.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # This script shuts the Gazebo PX4 software in the loop (SITL) simulation down again.
 

travis-docker-build.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 set -e