Update minimum required Apple versions

The versions supported by Rust was updated in rust-lang/rust#104385 to:
- macOS: 10.12 Sierra
- iOS: 10.0
- tvOS: 10.0
- watchOS: 5.0

Additionally, the armv7-apple-ios target was removed.

Finally, since v1.0.84 of `cc` (has been yanked, but 1.0.85 will include it as well), there is much better support for specifying the deployment target for Apple targets, so I've removed the corresponding code from objc-sys.

Cargo.toml

@@ -49,7 +49,6 @@ targets = [
     # iOS
     "aarch64-apple-ios",
     "x86_64-apple-ios",
-    # "armv7-apple-ios",
     # "i386-apple-ios",
     # GNUStep
     "x86_64-unknown-linux-gnu",