.travis.yml: Try to run in an LXD container

According to the build environment documentation we need to specify
an OS of Linux in order to get a container instead of a VM. This
should be faster in theory...

See: https://docs.travis-ci.com/user/reference/overview/

.travis.yml

@@ -11,6 +11,8 @@ before_script:
   - git clone https://github.com/alanorth/hugo-theme-bootstrap4-blog.git themes/hugo-theme-bootstrap4-blog
 script:
   - hugo
+os: linux
+arch: amd64
 dist: bionic
 
 # vim: ts=2 sw=2 et