Upgrade to Maven Sources Plugin 3.3.1

Closes gh-438

Author: snicoll

pom.xml

@@ -189,7 +189,7 @@
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-source-plugin</artifactId>
-					<version>3.3.0</version>
+					<version>3.3.1</version>
 				</plugin>
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>