Added Impala version date to benchmark report

Impala version line will look like this:
impalad version 2.1.0-cdh4-INTERNAL RELEASE (2014-12-09)

Change-Id: Id81c8561166d023024de3dd572e9ce4be9b8f01a
Reviewed-on: http://gerrit.sjc.cloudera.com:8080/5607
Reviewed-by: Alex Leblang <alex.leblang@cloudera.com>
Tested-by: jenkins