Fix the JUnit schema by changing 'skips' to 'skipped'

[mrijke]

The Jenkins xUnit plugin had a major update (https://wiki.jenkins.io/display/JENKINS/xUnit+Plugin, search for Version 2.0.0) that enforced strict checking of junit.xml files according to the XSD provided by Surefire / Ant Junit.

According to that XSD, skips is not allowed in <testsuite>, it should be skipped. This PR changes that in the runner module.