add retrofit to CI verified samples

Author: cbornet

pom.xml

@@ -426,6 +426,7 @@
                 <module>samples/client/petstore/java/default</module>
                 <module>samples/client/petstore/java/jersey2</module>
                 <module>samples/client/petstore/java/okhttp-gson</module>
+                <module>samples/client/petstore/java/retrofit</module>
                 <module>samples/client/petstore/scala</module>
                 <module>samples/server/petstore/spring-mvc</module>
                 <!--module>samples/client/petstore/objc</module-->