Default to latest installed iOS SDK by default

The iOS 7.1 SDK isn't available anymore in the current version of Xcode, so the script doesn't work without modification.
By setting IOS_SDK_VERSION to an empty value, the script will default to a symlink* which always points to the latest installed SDK.

* /Applications/Xcode.app/Contents/Developer/Platforms/iPhoneOS.platform/Developer/SDKs/iPhoneOS.sdk
Merge pull request #419 from sth519/patch-1

Default to latest installed iOS SDK by default