Update .travis to Xcode 9

Author: devxoul

.travis.yml

@@ -1,13 +1,13 @@
 os: osx
-osx_image: xcode8
+osx_image: xcode9
 sudo: false
 language: objective-c
 
 env:
-  - SDK="iphoneos10.0"
+  - SDK="iphoneos11.0"
 
 script:
-  - xcodebuild clean build -sdk $SDK -configuration Debug CODE_SIGN_IDENTITY=\"\" CODE_SIGNING_REQUIRED=NO
+  - xcodebuild clean build -sdk $SDK -configuration Debug CODE_SIGN_IDENTITY="" CODE_SIGNING_REQUIRED=NO
 
 before_install:
   - xcodebuild -showsdks