Fixed ant plugin issue

Author: jamtur01

riemann-java-client/pom.xml

@@ -70,13 +70,13 @@
                         <id>generate-sources</id>
                         <phase>generate-sources</phase>
                         <configuration>
-                            <tasks>
+                            <target>
                                 <mkdir dir="target/generated-sources"/>
                                 <exec executable="protoc">
                                     <arg value="--java_out=target/generated-sources"/>
                                     <arg value="src/main/proto/riemann/proto.proto"/>
                                 </exec>
-                            </tasks>
+                            </target>
                         </configuration>
                         <goals>
                             <goal>run</goal>