Update travis.yml to use newer xcode image and 10.11 for OSX build

.travis.yml

@@ -1,9 +1,9 @@
 language: objective-c
-osx_image: xcode6.4
+osx_image: xcode7.2
 before_script:
     - bundle install
 script:
     - xcodebuild -project SocketRocket.xcodeproj -scheme "SocketRocket" -sdk iphonesimulator -configuration Debug -PBXBuildsContinueAfterErrors=0 ACTIVE_ARCH_ONLY=0 build test
     - xcodebuild -project SocketRocket.xcodeproj -scheme "SocketRocket-iOS" -sdk iphonesimulator -configuration Debug -PBXBuildsContinueAfterErrors=0 ACTIVE_ARCH_ONLY=0 build
-    - xcodebuild -project SocketRocket.xcodeproj -scheme "SocketRocketOSX" -sdk macosx10.10 -configuration Debug -destination "platform=OS X" -PBXBuildsContinueAfterErrors=0 build
+    - xcodebuild -project SocketRocket.xcodeproj -scheme "SocketRocketOSX" -sdk macosx10.11 -configuration Debug -destination "platform=OS X" -PBXBuildsContinueAfterErrors=0 build
     - pod lib lint --verbose --fail-fast