Revert "RAT-290: Update animal sniffer maven plugin to 1.21"

Works locally but fails on Travis and ASF Jenkins - strange.

This reverts commit a558e0e.

pom.xml

@@ -446,7 +446,7 @@ agnostic home for software distribution comprehension and audit tools.
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>animal-sniffer-maven-plugin</artifactId>
-        <version>1.21</version>
+        <version>1.20</version>
         <executions>
           <execution>
             <!-- This checks the source code of our project -->