Update checkstyle to 8.18 to fix CVE-2019-9658

This was brought to my attention by Github's security alert feature.
Link: https://nvd.nist.gov/vuln/detail/CVE-2019-9658

pom.xml

@@ -57,7 +57,7 @@
                     <dependency>
                         <groupId>com.puppycrawl.tools</groupId>
                         <artifactId>checkstyle</artifactId>
-                        <version>8.11</version>
+                        <version>8.18</version>
                     </dependency>
                 </dependencies>
             </plugin>