[Travis] Build CI builds with Xcode 9, and test on iOS 11

Also, reduce the overall number of CI builds to speed up testing in response to Travis CI's reduction of the number of concurrent builds.

Author: mattrubin

.travis.yml

@@ -5,17 +5,14 @@ language: objective-c
 xcode_workspace: Authenticator.xcworkspace
 xcode_scheme: Authenticator
 
-osx_image: xcode8.3
+osx_image: xcode9
 
 env:
   - RUNTIME="iOS 9.0"  DEVICE="iPhone 4s"
-  - RUNTIME="iOS 9.1"  DEVICE="iPhone 5"
-  - RUNTIME="iOS 9.2"  DEVICE="iPhone 5s"
   - RUNTIME="iOS 9.3"  DEVICE="iPhone 6s"
-  - RUNTIME="iOS 10.0" DEVICE="iPhone 6s Plus"
-  - RUNTIME="iOS 10.1" DEVICE="iPhone SE"
-  - RUNTIME="iOS 10.2" DEVICE="iPhone 7"
+  - RUNTIME="iOS 10.0" DEVICE="iPhone SE"
   - RUNTIME="iOS 10.3" DEVICE="iPhone 7 Plus"
+  - RUNTIME="iOS 11.0" DEVICE="iPhone X"
 
 install:
   - git submodule update --init --recursive