.travis.yml: add a verbose build
.travis.yml: add a verbose build

I like to have a verbose build log for automation like Travis CI,
because it sometimes helps in diagnosing failures.

DonSheddow/num,rust-lang/num,rust-lang/num,rust-num/num-bigint,koverstreet/num,rust-num/num-integer,vks/num,rust-num/num,rust-num/num-traits,DonSheddow/num,rust-num/num-traits,rust-num/num-integer,rust-num/num,vks/num,koverstreet/num,rust-num/num-bigint