Remove bash-ism and use standard posix redirect in sshx.sh script.

Closes gh-274, thanks to github user juliantaylor for the fix!

IPython/kernel/scripts/ipcluster.py

@@ -411,7 +411,7 b' bsub < $1'
 
 sshx_template_prefix="""#!/bin/sh
 """
-sshx_template_suffix=""""$@" &> /dev/null &
+sshx_template_suffix=""""$@" > /dev/null 2>&1 &
 echo $!
 """