Tidy up the SBT project definition a little.

I was rereading it so I cleaned it up some.
Provide serif fallback for h3s in article body copy
Stage has changed on build agents
Stage has changed on build agents
Merge pull request #1027 from guardian/project-definition-tidy

Tidy up the SBT project definition a little.
Merge branch 'master' of github.com:guardian/frontend into article-breakpoints-tweaks
Show more cta is centered and font look better on desktop
Bump up bylines on tablets and above
Adjust spacing beneath headline