fix typo, must be 'classifier' to appear on pypi.python.org

The classifier info does not appear on pypi.python.org because of
the extra 's':  https://pypi.python.org/pypi/jenkins-job-builder

Also add a couple more classifiers.

Change-Id: I51ef51020bc0dc1d46a1e9049e7fa34be7b7d870
Merge "fix typo, must be 'classifier' to appear on pypi.python.org"