travis: add variant for WITH_BOOST_THREAD
travis: add variant for WITH_BOOST_THREAD

dagophil/vigra,dstoe/vigra,dstoe/vigra,funkey/vigra,dstoe/vigra,dagophil/vigra,funkey/vigra,funkey/vigra,dagophil/vigra,funkey/vigra,dstoe/vigra,funkey/vigra,dagophil/vigra,dstoe/vigra,funkey/vigra,funkey/vigra,dagophil/vigra,dagophil/vigra,dagophil/vigra,dstoe/vigra,dstoe/vigra