Cargo.toml: Fix backtrace crate to compatibile version

The latest backtrace release isn't compatible with 1.26.2. A patch for
this is underway in:

rust-lang/backtrace-rs#137

Though for now, let's just restrict the version since we should be
bumping our minimum rustc requirement soon-ish anyway.

Closes: #1713
Approved by: cgwalters

Author: jlebon

rust/Cargo.toml

@@ -20,6 +20,9 @@ c_utf8 = "0.1.0"
 systemd = "0.4.0"
 indicatif = "0.9.0"
 lazy_static = "1.1.0"
+# Drop this once we bump our minimum rustc requirement.
+# https://github.com/alexcrichton/backtrace-rs/pull/137
+backtrace = "=0.3.9"
 
 [lib]
 name = "rpmostree_rust"