.travis.yml: Update for Go 1.11

Golang now supports modules and I think Hugo's build system changed
to support this. Trying to port changes from the Hugo configuration
here:

https://github.com/gohugoio/hugo/blob/master/.travis.yml

.travis.yml

@@ -1,7 +1,11 @@
 language: go
 go:
-  - "1.10"
+  - "1.11"
-install: go get -v github.com/gohugoio/hugo
+install:
+  - mkdir -p $HOME/src
+  - git clone https://github.com/gohugoio/hugo.git $HOME/src/hugo
+  - cd $HOME/src/hugo
+  - go install
 before_script: git clone https://github.com/alanorth/hugo-theme-bootstrap4-blog.git themes/hugo-theme-bootstrap4-blog
 script: hugo
 sudo: false