Downgrade error-crate to get Sync bound

Needing a `Sync` bound, so we're downgrading
  temporarily to get it while keeping an eye on:
  rust-lang-deprecated/error-chain#134

Author: Anthony Nowell

Cargo.toml

@@ -25,7 +25,7 @@ serde_derive = { version = "0.9", optional = true }
 rustc-serialize = { version = "0.3.19", optional = true }
 base64 = "0.3.0"
 quick-error = "1.1.0"
-error-chain = "0.10.0"
+error-chain = "0.8.1"
 mime = "0.2.2"
 reqwest = "0.5"
 chrono = { version = "0.3", features = ["serde"] }