Revert maven-source-plugin to version 2.4 to speed up builds (#961)

* Speed up builds by using an older version of maven-source-plugin (2.4) that doesn't have speed issues on ubuntu (as does 3.0.0)

* Add comment to clarify maven-source-plugin downgrade

pom.xml

@@ -426,7 +426,12 @@
           <plugin>
             <groupId>org.apache.maven.plugins</groupId>
             <artifactId>maven-source-plugin</artifactId>
-            <version>3.0.0</version>
+            <!--
+              maven-source-plugin-3.0.0 exhibits the zipping behavior on ubuntu that was seen in
+              maven-jar-plugin with version < 2.5 (https://issues.apache.org/jira/browse/MJAR-188).
+              Until that is fixed we should stick to 2.4.
+            -->
+            <version>2.4</version>
             <executions>
               <execution>
                 <id>attach-sources</id>