Upgrade animal sniffer to 1.20

This addresses a bad class file issues seen when using an explicit
JAXB dependency.  Release notes:

https://github.com/mojohaus/animal-sniffer/releases
diff --git a/project/pom.xml b/project/pom.xml
index 814dab1..4fd0440 100644
--- a/project/pom.xml
+++ b/project/pom.xml
@@ -504,7 +504,7 @@
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>animal-sniffer-maven-plugin</artifactId>
-        <version>1.16</version>
+        <version>1.20</version>
         <executions>
           <execution>
             <phase>test</phase>