adding xcode 10 for Travis CI

Author: heyzooi

.travis.yml

@@ -14,9 +14,16 @@ matrix:
     - os: osx
       env: TYPE=macos
       osx_image: xcode9.4
+    - os: osx
+      env: TYPE=macos
+      osx_image: xcode10
+    - os: osx
+      env:
+        - TYPE=swiftpm
     - os: osx
       env:
         - TYPE=swiftpm
+      osx_image: xcode10
     - os: linux
       dist: trusty
       sudo: required