Deprecate defensive preview directory creation

It is no longer necessary to create the preview directory the first
time. It may have actually never been necessary, but there's no good way
to verify with older releases of the App Store version of Sketch, so
we'll just deprecate the creation for now.
Improve preview slice name comparison

Use NSString for preview slice name comparison. This lets us do case
insensitive comparison without a compound conditional, and it may
actually work right in different locales.
Create changelog