CentOS 7 RPM Specfile for a modern 25.1.x Unison version, based on unison248 (#1544239).
There are pre-built binary packages for CentOS 7 available on RaBe miscellaneous package repository, which can be installed as follows:
# Add RaBe Misc repository
curl -o /etc/yum.repos.d/home:radiorabe:misc.repo \
http://download.opensuse.org/repositories/home:/radiorabe:/misc/CentOS_7/home:radiorabe:misc.repo
# Install Unison
yum install unison-text
# Install the GTK frontend
yum install unison-gtk