I think the plugin directory layout in the README.md is incorrect. Could you try using the plugin directory layout documented in the Flume User Guide instead ?: http://archive.cloudera.com/cdh4/cdh/4/flume-ng/FlumeUserGuide.html#installing-third-party-plugins

your_plugins_dir/some_custom_name/lib/foo.jar

For example: If you specified /var/lib/flume-ng/plugins.d as the plugin directory (in CM's Flume Plugin directories configuration), then your jar location should be:

/var/lib/flume-ng/plugins.d/twitter-streaming/lib/flume-sources-1.0-SNAPSHOT.jar

thanks,
Mohit