travis: make build scripts use bash

These are development scripts, so no need for them to be fast or
super portable.  Switch them to bash rather than POSIX shell.

scripts/travis-build.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 set -ex
 

scripts/travis-checkout.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 set -ex
 
 get_depot_tools() {