downgrade xstream due to apparent bug with java 8
downgrade xstream due to apparent bug with java 8

vrthra/my-pitest,vrthra/my-pitest,august782/pitest,lkwg82/pitest,jacksonpradolima/pitest,roberth/pitest,lkwg82/pitest,vrthra/my-pitest,mzw/pitest,artspb/pitest,jacksonpradolima/pitest,august782/pitest,jasonmfehr/pitest,mzw/pitest,jacksonpradolima/pitest,hcoles/pitest,velo/pitest,velo/pitest,hcoles/pitest,artspb/pitest,jasonmfehr/pitest,lkwg82/pitest,velo/pitest,mzw/pitest,august782/pitest,roberth/pitest,artspb/pitest,jasonmfehr/pitest