For Travis CI - disable Mach-O universal binary build for 64-bit iOS.
When Travis CI is overloaded, the CI builds tend to get longer. Our iOS 32-bit and 64-bit CI build are usually finished very close to the 50 minutes time limit but may overshoot to pass the limit when Travis CI is overloaded. Using the xcpretty only helps in reducing the log size but it cannot help in this case. This commit disables again the universal binary build for the 64-bit iOS build when packaging the build artifact. The Mach-O universal binary build for 32-bit iOS build may be disabled too in the near future when we get more samples or the overloaded situation gets worse.
For Travis CI - switch CI build to use OS X build environment.