Downgrade to XCode 6.4 on Travis

facebookarchive/xctool#528 (comment)
mapbox/mapbox-gl-native#2238 (comment)

Author: anshumanf

.travis.yml

@@ -3,6 +3,6 @@ install:
 language      : objective-c
 xcode_project : ./ios/GithubNoteTaker.xcodeproj
 xcode_scheme  : GithubNoteTaker
-osx_image     : xcode7
-xcode_sdk     : iphonesimulator9.0
-xctool_args   : -destination 'platform=iOS Simulator,name=iPhone 6,OS=9.0'
+osx_image     : xcode6.4
+xcode_sdk     : iphonesimulator8.4
+xctool_args   : -destination 'platform=iOS Simulator,name=iPhone 6,OS=8.4'