travis: move to apt; throw some quiets around to make the output less verbose.
ruby is the default fallback language, so no need to set it explicitly.
Not much use of fast_finish for us - remove it for now.
Branches only isn't really needed as well.
Also, switch to bionic for number of reasons:
- sudo is enabled by default;
- preinstalled python3 and some of its modules;
- preinstalled bundler 2.* - no need to update gem.
Cache bundler for faster build.

Tired of constantly updating Gemfile.lcok, so remove it.
bionic already has the latest versions of required gems.

Small cosmetic fixes in README.md.

_config.yml: exclude some file we don't need.
Enable incremental regeneration again, for faster build.

_data/: whitespace and html escape fixups

Signed-off-by: Daniel Oaks <daniel@danieloaks.net>