Upgrade to findbugs-maven-plugin 3.0.4

This resolves an incompatibility with Maven 3.4.0.

pom.xml

@@ -205,7 +205,7 @@
       <plugin>
         <groupId>org.codehaus.mojo</groupId>
         <artifactId>findbugs-maven-plugin</artifactId>
-        <version>3.0.3</version>
+        <version>3.0.4</version>
         <configuration>
           <effort>Max</effort>
           <omitVisitors>FindNullDeref</omitVisitors>