Use `/usr/bin/env bash` instead of `/bin/bash` (#18)

Some systems may not have /bin/bash (NixOS for example)… Using
/usr/bin/env bash has the benefit of looking for whatever the
default version of the program is in your current environment

Signed-off-by: Vincent Demeester <vdemeest@redhat.com>

hack/boilerplate/add-boilerplate.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #

hack/boilerplate/boilerplate.sh.txt

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #

hack/update-codegen.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #

hack/update-deps.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #

hack/verify-codegen.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #

test/presubmit-tests.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2018 The Knative Authors
 #