Upgrade PaxExam version to 2.3.1

Upgrade Pax-Exam version from 2.3.0 to 2.3.1 due to a bug in combination with latest Guava library upgraded version (from 10.0.1 to 16.0.1).

Documented bug and solution:
* Karaf bug: [KARAF-1207](https://issues.apache.org/jira/browse/KARAF-1207)
* PaxExam fixed release: [PAXEXAM-353](https://ops4j1.jira.com/browse/PAXEXAM-353)