Build with a newer version of XCode.
Build with a newer version of XCode.

Travis CI by default uses XCode 7.3 on Mac OS X, which does not support
the C++11 thread_local keyword expected by Subzero. Request XCode 8 GM
instead.

Change-Id: I0afa7804997ff5b5a6bbfe7829ba6134fe3d54ca
Reviewed-on: https://swiftshader-review.googlesource.com/11548
Reviewed-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>
Tested-by: Nicolas Capens <51edc787c30f24c4904e8ffbb5523c3a9f7a451d@google.com>

bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader