Travis: Build against Xcode 10.1 & .2

... no more 9, thought that should work as well if the
language version is lowered!

Author: helje5

.travis.yml

@@ -20,7 +20,7 @@ matrix:
           env:  SWIFT_SNAPSHOT_NAME="https://swift.org/builds/swift-5.0-release/ubuntu1404/swift-5.0-RELEASE/swift-5.0-RELEASE-ubuntu14.04.tar.gz"
           sudo: required
         - os:   osx
-          osx_image: xcode9.2
+          osx_image: xcode10.1
         - os:   osx
           osx_image: xcode10.2