jenkins: add debugging output to shell script

When we migrated the Jenkins commands into Git, the exact commands are
no longer visible unless we run with "-x".

Add "set -x" to the script to enable debugging.

Signed-off-by: Ken Dreyer <ken.dreyer@inktank.com>
Merge pull request #202 from kdreyer-inktank/jenkins

jenkins: add debugging output to shell script