travis.yml: Bump osx_image to xcode8.3
travis.yml: Bump osx_image to xcode8.3

jjthrash/kakoune,casimir/kakoune,lenormf/kakoune,danr/kakoune,Somasis/kakoune,danr/kakoune,alexherbo2/kakoune,occivink/kakoune,mawww/kakoune,lenormf/kakoune,alexherbo2/kakoune,jjthrash/kakoune,Somasis/kakoune,alexherbo2/kakoune,lenormf/kakoune,jjthrash/kakoune,occivink/kakoune,mawww/kakoune,casimir/kakoune,lenormf/kakoune,danr/kakoune,jkonecny12/kakoune,casimir/kakoune,jkonecny12/kakoune,alexherbo2/kakoune,occivink/kakoune,danr/kakoune,jkonecny12/kakoune,mawww/kakoune,Somasis/kakoune,occivink/kakoune,jkonecny12/kakoune,mawww/kakoune,casimir/kakoune,Somasis/kakoune,jjthrash/kakoune