Update macos deps

[j4qfrost]

This is to allow for building coverage files per this issue servo/core-foundation-rs#357

[aclysma]

I'm open to bumping the macOS-specific dependencies that are already locked to specific versions. (metal/cocoa)

For the skia and ash dependencies, upstream changes are unlikely to break skulpin because they are either stable (ash) or very little of it is used (sdl2). There was more discussion of the rationale here: #42

I'm open to considering new solutions to this. I don't think the current one is ideal but I like it better than requiring a specific version of ash/skia as I was doing before.

[j4qfrost]

My only concern might be that skia has a version bump before the binary is uploaded to git, which triggers a build from source. Though this is more like a mild annoyance since pragmatrix gets the binary up fairly close to pushing the source.