Update checkstyle (#80)

* Fixes CVE-2019-9658
diff --git a/pom.xml b/pom.xml
index 67944f4..b3109fd 100644
--- a/pom.xml
+++ b/pom.xml
@@ -240,7 +240,7 @@
           <dependency>
             <groupId>com.puppycrawl.tools</groupId>
             <artifactId>checkstyle</artifactId>
-            <version>8.17</version>
+            <version>8.18</version>
           </dependency>
         </dependencies>
         <executions>