v0.2.0 doesnt build on release channel (1.3)

[AutomatedTester]

Building a project with 0.2.0 doesn't compile.

toml

[package]
name = "learn"
version = "0.1.0"
authors = ["AutomatedTester <dburns@mozilla.com>"]

[dependencies]

argparse = "*"
rustbar = "0.2.0"

results in

dburns in ~/development/learn on master ● λ cargo build
   Compiling argparse v0.2.0
   Compiling rustbar v0.2.0
/Users/dburns/.cargo/registry/src/github.com-0a35038f75765ae4/rustbar-0.2.0/src/lib.rs:2:1: 2:18 error: #[feature] may not be used on the stable release channel
/Users/dburns/.cargo/registry/src/github.com-0a35038f75765ae4/rustbar-0.2.0/src/lib.rs:2 #![feature(libc)]
                                                                                         ^~~~~~~~~~~~~~~~~
error: aborting due to previous error
Build failed, waiting for other jobs to finish...
Could not compile `rustbar`.

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