Fix Travis link in README.md
Fix Travis link in README.md

Nitpicking, but it's nicer to just be able to click it...

Cheers
pwwpche/scalac-scapegoat-plugin,jasonchaffee/scalac-scapegoat-plugin,sksamuel/scalac-scapegoat-plugin,sksamuel/scapegoat,vdichev/scapegoat