travis: Set GO111MODULE=on for mage install
travis: Set GO111MODULE=on for mage install

See #5115

anthonyfok/hugo,mdhender/hugo,mdhender/hugo,gohugoio/hugo,anthonyfok/hugo,anthonyfok/hugo,mdhender/hugo,fedelibre/hugo,gohugoio/hugo,mdhender/hugo,fedelibre/hugo,gohugoio/hugo,fedelibre/hugo,fedelibre/hugo