Update commons-parent 50 to 51

Allows bundling LICENSE and NOTICE in the jar files.
diff --git a/pom.xml b/pom.xml
index 250fd89..9590f66 100644
--- a/pom.xml
+++ b/pom.xml
@@ -21,7 +21,7 @@
   <parent>
     <groupId>org.apache.commons</groupId>
     <artifactId>commons-parent</artifactId>
-    <version>50</version>
+    <version>51</version>
   </parent>
 
   <artifactId>commons-numbers-parent</artifactId>