Update Travis configuration
Update Travis configuration

- Update to Xcode version 10.2
- Remove linting stage (SwiftLint is run during the build process)
- Remove unused Slack notifications (already integrated via GitHub)

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