Cache Hugo on build job

[johnjago]

https://docs.travis-ci.com/user/caching/

The version of Hugo in the apt repository is terribly outdated, and the does not seem to be a PPA for it.

If it can be cached after compiling, it won't have to be compiled from source every time, which would cut the build time in half, or more.

[johnjago]

Instead of caching, I added the binary for Hugo to the repository.

Result: Average build time decreased from 4 minutes to 1 minute.