add deprecation warning for CentOS 7 / RHEL 7

With this patch:

    sh -c install.sh
    # Executing docker install script, commit:

    DEPRECATION WARNING
        This Linux distribution (centos 7) reached end-of-life and is no longer supported by this script.
        No updates or security fixes will be released for this distribution, and users are recommended
        to upgrade to a currently maintained version of centos.

    Press Ctrl+C now to abort this script, or wait for the installation to continue.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

install.sh

@@ -473,6 +473,9 @@ do_install() {
 	# Print deprecation warnings for distro versions that recently reached EOL,
 	# but may still be commonly used (especially LTS versions).
 	case "$lsb_dist.$dist_version" in
+		centos.7|rhel.7)
+			deprecation_notice "$lsb_dist" "$dist_version"
+			;;
 		debian.stretch|debian.jessie)
 			deprecation_notice "$lsb_dist" "$dist_version"
 			;;