Revert "Temporarily disabled Maven Checkstyle plugin."

This reverts commit b2d929f8.

pom.xml

@@ -377,7 +377,6 @@
 				<artifactId>aspectj-maven-plugin</artifactId>
 				<version>1.4</version>
 			</plugin>
-			<!--
 			<plugin>
 				<groupId>org.apache.maven.plugins</groupId>
 				<artifactId>maven-checkstyle-plugin</artifactId>
@@ -386,7 +385,6 @@
 					<configLocation>ARSnova-checkstyle-checker.xml</configLocation>
 				</configuration>
 			</plugin>
-			-->
 			<plugin>
 				<groupId>org.apache.tomcat.maven</groupId>
 				<artifactId>tomcat7-maven-plugin</artifactId>