Fix jenkins.sh to report test failures

When I added the task to check for bad time handling in 6db125f8bfa7241d50bff876ef736e45cc0763e0
it meant that the last command was no longer specs/cucumber, so the exit status
was zero as long as the time check passed. This meant that Jenkins CI thought
the build was successful.

Rather than move the time-handling around, let's make it more robust and tell
the script to exit (with non-zero) if any command returns non-zero.
Merge pull request #246 from alphagov/fix_jenkins_reporting_failures

Fix jenkins.sh to report test failures