Fix LOGSTASH-1936

Plugin script failed because it was trying to gzip -dc ${TARGETDIR}${FILENAME},

This was missing a / between the directory and the filename, so it was trying to
uncompress: /opt/logstash/logstash-1.4.0.beta1/vendor/logstashlogstash-contrib-1.4.0.beta1.tar.gz

Changed to uncompress ${TARGET}, which is defined as: ${TARGETDIR}/${FILENAME}