opal-ci: Use ubuntu:rolling for Ubuntu latest image
This updates the Ubuntu 'latest' to use ubuntu:rolling, which is
the most recent release. It turns out that ubuntu:latest is actually the
latest LTS (18.04).
-FROM ubuntu:latest
+FROM ubuntu:rolling
ENV DEBIAN_FRONTEND noninteractive
RUN apt-get update -qq
RUN if [ `arch` != "ppc64le" ]; then apt-get update -qq && apt-get install -y gcc-powerpc64le-linux-gnu; fi
