Fix up Xcode 8.3.x support.

- Xcode 8.3.3 bumped the simulators to 10.3.1, so 10.3 fails.  Just use
  the "latest" naming instead to avoid issues.
- Also switch to just testing an old 32bit build and new 64bit build as
  there isn't any code in protobuf that should care about form facts.
  (Looks like the iPad Pro simulator names aren't always constantly, on
  some developer machines they are "iPad Pro (9.7 inch)" while on others
  they are "iPad Pro (9.7-inch)"; so just avoid using them.)

objectivec/DevTools/full_mac_build.sh

@@ -276,9 +276,7 @@ if [[ "${DO_XCODE_IOS_TESTS}" == "yes" ]] ; then
     8.3* )
       XCODEBUILD_TEST_BASE_IOS+=(
           -destination "platform=iOS Simulator,name=iPhone 4s,OS=8.1" # 32bit
-          -destination "platform=iOS Simulator,name=iPhone 7,OS=10.3" # 64bit
-          -destination "platform=iOS Simulator,name=iPad 2,OS=8.1" # 32bit
-          -destination "platform=iOS Simulator,name=iPad Pro (9.7 inch),OS=10.3" # 64bit
+          -destination "platform=iOS Simulator,name=iPhone 7,OS=latest" # 64bit
       )
       ;;
     * )