fix(functions): include Implementation-Version key in invoker package manifest (#221)

Author: garethgeorge

invoker/core/pom.xml

@@ -165,6 +165,8 @@
           <archive>
             <manifest>
               <mainClass>com.google.cloud.functions.invoker.runner.Invoker</mainClass>
+              <addDefaultImplementationEntries>true</addDefaultImplementationEntries>
+              <addDefaultSpecificationEntries>true</addDefaultSpecificationEntries>
             </manifest>
           </archive>
         </configuration>
@@ -184,4 +186,4 @@
       </plugin>
     </plugins>
   </build>
-</project>
+</project>