Bump log4j version (used for testing)

Author: jdhoek

pom.xml

@@ -50,7 +50,7 @@
     <slf4j.version>1.7.21</slf4j.version>
 
     <!-- For testing only. -->
-    <log4j.version>2.7</log4j.version>
+    <log4j.version>2.13.2</log4j.version>
     <hamcrest.optional>1.0</hamcrest.optional>
   </properties>