Updated README to remove travis and add buildkite.
Updated README to remove travis and add buildkite.

slightair/SwiftGraphics,hejunbinlan/SwiftGraphics,sdesimone/SwiftGraphics,SheffieldKevin/SwiftGraphics,Pretz/SwiftGraphics,marinehero/SwiftGraphics,Pretz/SwiftGraphics,SheffieldKevin/SwiftGraphics,sdesimone/SwiftGraphics,marinehero/SwiftGraphics,sdesimone/SwiftGraphics,sdesimone/SwiftGraphics,hejunbinlan/SwiftGraphics,slightair/SwiftGraphics,slightair/SwiftGraphics,hejunbinlan/SwiftGraphics,hejunbinlan/SwiftGraphics,slightair/SwiftGraphics,Pretz/SwiftGraphics,Pretz/SwiftGraphics,marinehero/SwiftGraphics,marinehero/SwiftGraphics,SheffieldKevin/SwiftGraphics