Lock backtrace crate to 0.3.9.

The current latest (0.3.11)` is not compatible with Rust 1.18.0.
Pending <rust-lang/backtrace-rs#137>

Author: azriel91

Cargo.toml

@@ -15,7 +15,7 @@ path = "./failure_derive"
 
 [dependencies.backtrace]
 optional = true
-version = "0.3.3"
+version = "=0.3.9"
 
 [workspace]
 members = [".", "failure_derive"]