update JUnit dependency to 4.13.1

Closes #6
diff --git a/pom.xml b/pom.xml
index 7b417a3..d3374c7 100644
--- a/pom.xml
+++ b/pom.xml
@@ -195,7 +195,7 @@
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
-      <version>4.13</version>
+      <version>4.13.1</version>
       <scope>test</scope>
     </dependency>
   </dependencies>