SnapKit supports legacy platforms but only through manual integration of the code:
Legacy Platforms are:
You must integrate SnapKit manually by dragging the source code into your project and in your project you must add -DSNAPKIT_DEPLOYMENT_LEGACY to OTHER_SWIFT_FLAGS in your target's build settings.
Unfortunately there is no other way to ensure that iOS 8.0/9.0 targets have no warnings and iOS 7.0 still works.