travis.yml: add a newer version of boost
travis.yml: add a newer version of boost

lezorich/mlpack,Azizou/mlpack,ranjan1990/mlpack,theranger/mlpack,erubboli/mlpack,Azizou/mlpack,chenmoshushi/mlpack,ajjl/mlpack,palashahuja/mlpack,thirdwing/mlpack,thirdwing/mlpack,datachand/mlpack,bmswgnp/mlpack,chenmoshushi/mlpack,lezorich/mlpack,ajjl/mlpack,darcyliu/mlpack,bmswgnp/mlpack,palashahuja/mlpack,trungda/mlpack,datachand/mlpack,ranjan1990/mlpack,BookChan/mlpack,Azizou/mlpack,ajjl/mlpack,stereomatchingkiss/mlpack,stereomatchingkiss/mlpack,minhpqn/mlpack,BookChan/mlpack,chenmoshushi/mlpack,minhpqn/mlpack,trungda/mlpack,theranger/mlpack,bmswgnp/mlpack,trungda/mlpack,ersanliqiao/mlpack,BookChan/mlpack,thirdwing/mlpack,lezorich/mlpack,erubboli/mlpack,erubboli/mlpack,darcyliu/mlpack,ersanliqiao/mlpack,minhpqn/mlpack,datachand/mlpack,palashahuja/mlpack,ersanliqiao/mlpack,ranjan1990/mlpack,theranger/mlpack,stereomatchingkiss/mlpack,darcyliu/mlpack