Switch to /bin/bash everywhere

The referenced issue points out portability issues with 'echo -n' in
/bin/sh scripts. As we use some bash constructs already, make this
explicit in all scripts. There are several issues reported by
'checkbashisms -f -n -p' so do the simple fix for now.

Issue: #74
Signed-off-by: David Sterba <dsterba@suse.com>

btrfs-balance.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2017 SUSE LINUX GmbH, Nuernberg, Germany.
 

btrfs-defrag.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2018 SUSE LINUX GmbH, Nuernberg, Germany.
 

btrfs-scrub.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2018 SUSE LINUX GmbH, Nuernberg, Germany.
 

btrfs-trim.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2018 SUSE LINUX GmbH, Nuernberg, Germany.
 

btrfsmaintenance-refresh-cron.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 #
 # Copyright (c) 2017 SUSE LINUX GmbH, Nuernberg, Germany.
 

dist-install.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 # usage: $0 [sysconfdir]
 #
 # Install configuration template, documentation and scripts. Target path is

prepare-release.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 base=btrfsmaintenance
 version=$(grep -i ^version: ${base}.spec | awk '{print $2}')