Upgrade to error-prone 2.0.4

Release notes:

https://groups.google.com/forum/#!topic/error-prone-announce/syl8dKP_Aow
https://groups.google.com/forum/#!topic/error-prone-announce/iZI0yixYDms
https://groups.google.com/forum/#!topic/error-prone-announce/v7aBgfhH-u8
https://groups.google.com/forum/#!topic/error-prone-announce/-f6Cv6jKvig

pom.xml

@@ -112,6 +112,11 @@
             <artifactId>plexus-compiler-javac-errorprone</artifactId>
             <version>2.5</version>
           </dependency>
+          <dependency>
+            <groupId>com.google.errorprone</groupId>
+            <artifactId>error_prone_core</artifactId>
+            <version>2.0.4</version>
+          </dependency>
         </dependencies>
       </plugin>
       <plugin>