Update travis CI config to use Xcode 10
Update travis CI config to use Xcode 10

It still uses macOS 10.13 for now but hopefully it will move to macOS 10.14 once they update their images.

barijaona/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,Eitot/vienna-rss,ViennaRSS/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss,ViennaRSS/vienna-rss,barijaona/vienna-rss,barijaona/vienna-rss,Eitot/vienna-rss,josh64x2/vienna-rss,barijaona/vienna-rss,ViennaRSS/vienna-rss