Revert "IPHONEOS_DEPLOYMENT_TARGET = 7.0."

This reverts commit 48f95e58017eb1f88ae4e2b3cb5db057a6225f38.

Various bits of code don't compile with the target set to 6.0 or greater,
and this seems to be deliberate upstream.  See
https://github.com/ReactiveCocoa/ReactiveCocoa/issues/204,
(though that's old and maybe we could reopen the argument).