.travis.yml: Several small improvements

 * Reduce the iOS deployment target to 9.0.
 * Define _Nonnull, _Nullable and _Null_unspecified for Xcode 6.
   Xcode 6 used __nonnull, __nullable and __null_unspecified instead,
   while already defining __has_feature(nullability).
 * Always set WRAPPER=true for iOS simulator builds.
   Old versions of Xcode generated binaries that target the simulator
   but could be executed outside of it, resulting in the tests being run
   as configure noticed it can run binaries, but the tests failing
   because they expected an iOS environment.