Enable checkstyle

Author: dongjoon-hyun

pom.xml

@@ -2786,7 +2786,7 @@
         <artifactId>maven-checkstyle-plugin</artifactId>
         <version>3.1.0</version>
         <configuration>
-          <failOnViolation>false</failOnViolation>
+          <failOnViolation>true</failOnViolation>
           <includeTestSourceDirectory>true</includeTestSourceDirectory>
           <sourceDirectories>
             <directory>${basedir}/src/main/java</directory>