How to get pg_archivecleanup on Amazon Linux 2014.03?
With the help of this AWS forum thread, I realized that this line in /etc/yum.conf

exclude=postgresql*


excludes not only updates but installs.  Once I commented that out (temporarily), I was able to use

sudo yum install postgresql-contrib


to install postgresql-contrib directly from the Amazon repo--no need for changing the repository or the operating system.