Simplify travis-ci build

This change avoids the need to copy the source out of the GOPATH by
applying GO111MODULE=on as an environment variable. Additionally, avoids
issues with parallel module downloads (see golang/go#26794 and
golang/go#27372) by running 'go mod download' during the install step.

.travis.yml

@@ -11,16 +11,15 @@ go:
 os:
  - linux
  - osx
+env:
+- GO111MODULE=on
 matrix:
  allow_failures:
  - go: tip
  fast_finish: true
 install:
- - mkdir -p $HOME/src
- - mv $HOME/gopath/src/github.com/gohugoio/hugo $HOME/src
- - export TRAVIS_BUILD_DIR=$HOME/src/hugo
- - cd $HOME/src/hugo
  - go get github.com/magefile/mage
+ - go mod download
 script:
 - mage -v test
 - mage -v check