Fix #1: Better to stick to a single common shell

Referred a couple other version managers. The advantage is not having to
maintain the scripts for other shells (bash comes by default everywhere
I've seen).

asdf.sh

@@ -1,6 +1,4 @@
-#!/usr/bin/env sh
-
-#TODO check for ${BASH_SOURCE[0]} or use $0
+#!/usr/bin/env bash
 
 current_script_path=${BASH_SOURCE[0]}
 asdf_dir=$(cd $(dirname $current_script_path); echo $(pwd))

bin/asdf

@@ -1,4 +1,4 @@
-#!/usr/bin/env sh -l
+#!/usr/bin/env bash
 
 source $(dirname $(dirname $0))/lib/utils.sh
 

bin/private/asdf-exec

@@ -1,4 +1,4 @@
-#!/usr/bin/env sh
+#!/usr/bin/env bash
 
 source $(dirname $(dirname $(dirname $0)))/lib/utils.sh