Update .travis.yml to build iOS sample project

Summary: Closes https://github.com/facebook/yoga/pull/472

Differential Revision: D4688993

Pulled By: emilsjolander

fbshipit-source-id: 9b5a16866c4258245a50582aa1110d54139a7ec2

.travis.yml

@@ -40,7 +40,10 @@ before_install:
   # iOS
   - |
     if [[ $TARGET = "ios" ]]; then
-      brew outdated xctool || brew upgrade xctool
+      brew outdated xctool || brew upgrade xctool;
+      gem install xcpretty --no-document --quiet;
+      gem install cocoapods --pre --no-document --quiet;
+      pod install --project-directory=YogaKit/YogaKitSample/;
     fi
 
   # Emscripten (used for js tests)
@@ -102,7 +105,8 @@ script:
   # iOS
   - |
     if [[ $TARGET = "ios" ]]; then
-      buck test --verbose 0 //YogaKit:YogaKitTests --config cxx.default_platform=iphonesimulator-x86_64
+      buck test --verbose 0 //YogaKit:YogaKitTests --config cxx.default_platform=iphonesimulator-x86_64 &&
+      xcodebuild build -workspace YogaKit/YogaKitSample/YogaKitSample.xcworkspace -scheme YogaKitSample -sdk iphonesimulator | xcpretty -c
     fi
 
   # Android