Update Travis to Xcode 10.2, which also uses macOS Mojave

Update to Xcode 10.2 beta, which is still in beta but the VM now use
Mojave which is what we want.

Author: ychin

.travis.yml

@@ -8,7 +8,7 @@ osx_image:
   - xcode7.3
   - xcode8.3
   - xcode9.4
-  - xcode10.1
+  - xcode10.2
 
 compiler:
   - clang