tests: Update travis-ci tests
This updates the travis-ci tests to run on CentOS 8 instead of 7.
-FROM centos:7
+FROM centos:8
ARG TRAVIS_COMMIT_RANGE
ENV TRAVIS_COMMIT_RANGE=$TRAVIS_COMMIT_RANGE
ENV PYTHON_BINARY=python3
-RUN yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
+RUN yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
RUN yum install -y git ${PYTHON_BINARY}
COPY . /ohpc
