Update iOS travis to make it pass (#133)

Summary:
It updates the `xcodebuild` command with the sdk argument
Closes https://github.com/facebook/Sonar/pull/133

Reviewed By: danielbuechele

Differential Revision: D8732525

Pulled By: priteshrnandgaonkar

fbshipit-source-id: 3aafa44cf00f49e507246ec25329a58749025cb4

.travis.yml

@@ -1,5 +1,5 @@
 os: osx
-osx_image: xcode9.3
+osx_image: xcode9.4
 
 matrix:
   include:
@@ -58,4 +58,5 @@ matrix:
 
       script:
         - cd iOS/Sample
-        - xcodebuild clean build -workspace Sample.xcworkspace -scheme Pods-Sample -sdk iphonesimulator11.2
+        - xcodebuild -showsdks
+        - xcodebuild clean build -workspace Sample.xcworkspace -scheme Pods-Sample -sdk iphonesimulator11.4

docs/getting-started.md

@@ -83,7 +83,6 @@ target 'MyApp' do
         end
     end
 end
-
 ```
 
 and install the dependencies by running `pod install`. When you open the Xcode workspace file for your app, you now can import and initialize Sonar in your AppDelegate.