Add Checkstyle to verify target

pom.xml

@@ -44,6 +44,15 @@
         <groupId>org.apache.maven.plugins</groupId>
         <artifactId>maven-checkstyle-plugin</artifactId>
         <version>2.12.1</version>
+        <executions>
+          <execution>
+            <id>check</id>
+            <phase>verify</phase>
+            <goals>
+              <goal>check</goal>
+            </goals>
+          </execution>
+        </executions>
         <configuration>
           <configLocation>src/main/resources/checkstyle.xml</configLocation>
           <headerLocation>src/main/resources/copyright_header.txt</headerLocation>