Upgrade to CheckStyle Maven Plugin 3.1.0

Closes gh-109

Author: philwebb

Diff for: pom.xml

@@ -81,12 +81,12 @@
 				<plugin>
 					<groupId>org.apache.maven.plugins</groupId>
 					<artifactId>maven-checkstyle-plugin</artifactId>
-					<version>3.0.0</version>
+					<version>3.1.0</version>
 					<dependencies>
 						<dependency>
 							<groupId>com.puppycrawl.tools</groupId>
 							<artifactId>checkstyle</artifactId>
-							<version>8.18</version>
+							<version>8.21</version>
 						</dependency>
 					</dependencies>
 				</plugin>