Upgrade to maven-site-plugin 3.9.1

Closes gh-243

Author: philwebb

pom.xml

@@ -153,7 +153,7 @@
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-site-plugin</artifactId>
-					<version>3.7.1</version>
+					<version>3.9.1</version>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>