Generate source tar files instead of zip

Generate source tar.gz files instead of zip files to be
consistent with the binary release.

https://jira.apache.org/jira/projects/CRAIL/issues/CRAIL-54

Signed-off-by: Jonas Pfefferle <pepperjo@apache.org>
diff --git a/pom.xml b/pom.xml
index bbe65da..c5ef061 100644
--- a/pom.xml
+++ b/pom.xml
@@ -40,6 +40,7 @@
   <properties>
     <build.copyDependenciesPhase>package</build.copyDependenciesPhase>
     <hadoop.version>2.7.3</hadoop.version>
+    <sourceReleaseAssemblyDescriptor>source-release-tar</sourceReleaseAssemblyDescriptor>
   </properties>
 
   <profiles>