Upgrade rust/crates/futures-channel to 0.3.15

Test: make
Change-Id: I09aa640ae00a1055141833a6ff497d4dab47fc97
diff --git a/Cargo.toml.orig b/Cargo.toml.orig
index 9a33320..fae78a0 100644
--- a/Cargo.toml.orig
+++ b/Cargo.toml.orig
@@ -1,7 +1,7 @@
 [package]
 name = "futures-channel"
 edition = "2018"
-version = "0.3.13"
+version = "0.3.15"
 authors = ["Alex Crichton <alex@alexcrichton.com>"]
 license = "MIT OR Apache-2.0"
 repository = "https://github.com/rust-lang/futures-rs"
@@ -17,15 +17,14 @@
 alloc = ["futures-core/alloc"]
 sink = ["futures-sink"]
 
-# Unstable features
-# These features are outside of the normal semver guarantees and require the
-# `unstable` feature as an explicit opt-in to unstable API.
-unstable = ["futures-core/unstable"]
-cfg-target-has-atomic = ["futures-core/cfg-target-has-atomic"]
+# These features are no longer used.
+# TODO: remove in the next major version.
+unstable = []
+cfg-target-has-atomic = []
 
 [dependencies]
-futures-core = { path = "../futures-core", version = "0.3.13", default-features = false }
-futures-sink = { path = "../futures-sink", version = "0.3.13", default-features = false, optional = true }
+futures-core = { path = "../futures-core", version = "0.3.15", default-features = false }
+futures-sink = { path = "../futures-sink", version = "0.3.15", default-features = false, optional = true }
 
 [dev-dependencies]
 futures = { path = "../futures", default-features = true }