Fix bash paths in shebangs

"/bin/bash" is a Linuxism.  "/usr/bin/env bash" is portable.

scripts/build-docs.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2014 Vladimir Prus
 # Distributed under the Boost Software License, Version 1.0.

scripts/nightly.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2004, 2005, 2006 Vladimir Prus 
 # Distributed under the Boost Software License, Version 1.0. 

scripts/roll.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 # Copyright 2004 Aleksey Gurtovoy
 # Copyright 2006 Rene Rivera

scripts/to_merge.sh

@@ -1,4 +1,4 @@
-#!/bin/bash
+#!/usr/bin/env bash
 
 for python in build/*.py; do
     jam="${python%.py}.jam"