Use /bin/bash instead of /bin/sh in scripts

`/bin/sh` isn't always an alias of `/bin/bash`, so bash-specific syntax broke the scripts.
(like 777a9ae6e4/scripts/_get-test-directories.sh (L7))

scripts/_get-test-directories.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 TEST_DIRS=""

scripts/grant-npm-owner.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 read -p "Username: " username

scripts/test-cov.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 testDirs=`TEST_TYPE=cov scripts/_get-test-directories.sh`

scripts/test.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -e
 
 if [ -z "$TEST_GREP" ]; then