Explicitly call bash for bash-specific scripts

Thanks to Solaris-using Brian Cameron for pointing out that our
shell scripts are bash-specific. We'd be glad if someone cared to
rewrite them to not require bash, but for now let's have truth in
advertising at least.

perf/cairo-perf-diff

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

src/check-def.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 LANG=C
 

src/check-plt.sh

@@ -1,4 +1,4 @@
-#!/bin/sh
+#!/bin/bash
 
 LANG=C