Removed build warnings when using iOS 8 or 10.10 SDKs for the Date code.

@segiddins I've removed the build warnings for the iOS 8 and OS X 10.10 SDKs.

I have code for the `RKISO8601DateFormatter` to deal with those build warnings as well, but I can't figure out how to initiate a pull request for that code, since @blakewatters original repo doesn't have the 'RK'-prefixed version of the formatter and I don't see it in any convenient location in the RK Repo.

Please advise where/how I should open up a PR for RKISO8601DateFormatter. Thanks.