restrict travis to master only to avoid double testing of six branch with pullrequest
restrict travis to master only to avoid double testing of six branch with pullrequest

RDFLib/rdflib,RDFLib/rdflib,armandobs14/rdflib,marma/rdflib,armandobs14/rdflib,avorio/rdflib,marma/rdflib,yingerj/rdflib,ssssam/rdflib,ssssam/rdflib,dbs/rdflib,RDFLib/rdflib,dbs/rdflib,avorio/rdflib,RDFLib/rdflib,avorio/rdflib,marma/rdflib,ssssam/rdflib,marma/rdflib,avorio/rdflib,ssssam/rdflib,dbs/rdflib,yingerj/rdflib,armandobs14/rdflib,armandobs14/rdflib,yingerj/rdflib,dbs/rdflib,yingerj/rdflib