Resolve home dirs to absolute paths in whisky_darwin.go

[jtatum]

os.stat requires an absolute path or it fails. On my system, os.UserHomeDir() returns ~. Similarly, my whisky plist has defaultBottleLocation = "~/Library/Containers/com.isaacmarovitz.Whisky/Bottles";. This change dereferences both of those tildes to absolute paths so Whisky is properly detected.