Upgrade error_prone_core to 2.3.1

and plexus-compiler-javac-errorprone to 2.8.4.

Change-Id: I36487ce53e70b54d7a04292e666540224c107b43
Signed-off-by: David Pursehouse <david.pursehouse@gmail.com>

pom.xml

@@ -311,14 +311,14 @@
             <dependency>
               <groupId>org.codehaus.plexus</groupId>
               <artifactId>plexus-compiler-javac-errorprone</artifactId>
-              <version>2.8.2</version>
+              <version>2.8.4</version>
             </dependency>
             <!-- override plexus-compiler-javac-errorprone's dependency on
                  Error Prone with the latest version -->
             <dependency>
               <groupId>com.google.errorprone</groupId>
               <artifactId>error_prone_core</artifactId>
-              <version>2.2.0</version>
+              <version>2.3.1</version>
             </dependency>
           </dependencies>
         </plugin>