I have found a working solution for Travis:
before_install:
# This workaround is required to avoid libstdc++ errors while running "extended" hugo with SASS support.
- wget -q -O libstdc++6 http://security.ubuntu.com/ubuntu/pool/main/g/gcc-5/libstdc++6_5.4.0-6ubuntu1~16.04.10_amd64.deb
- sudo dpkg --force-all -i libstdc++6
install:
- wget -q -O hugo.deb https://github.com/gohugoio/hugo/releases/download/v0.46/hugo_extended_0.46_Linux_64bit.deb
- sudo dpkg -i hugo.deb