update PINRemoteImage for xcode 12, prepare release of 5.0.4

[mr-fixit]

This PR does 3 things:

• reverts my previous commit that .gitignored and removed Carthage/Checkouts. it broke scripts/carthage-update. see https://github.com/Carthage/Carthage/issues/1153.
• updates to PINRemoteImage 3.0.1 for compatibility with Xcode 12
• updates the docs, versions, etc for 5.0.4

I did NOT commit the Carthage/Checkouts/libwebp that PINRemoteImage 3.0.1 brought in, but I did commit the changes to the pre-existing folders. This will allow folks to build after a git clone or git checkout, without needing to run carthage.

[mr-fixit]

could someone approve this, so i can publish 5.0.4?