Use git dependency on failure 1.0 for right now

Cargo.toml

@@ -26,7 +26,7 @@ builtin-lua = ["cc"]
 
 [dependencies]
 libc = { version = "0.2" }
-failure = { version = "0.1.1" }
+failure = { git = "https://github.com/rust-lang-nursery/failure", version = "1.0" }
 compiletest_rs = { version = "0.3", optional = true }
 
 [build-dependencies]

src/error.rs

@@ -185,9 +185,7 @@ impl failure::Fail for Error {
     fn cause(&self) -> Option<&failure::Fail> {
         match *self {
             Error::CallbackError { ref cause, .. } => Some(cause.as_ref()),
-            // Error::cause simply returns the contained Fail type, which we are already displaying
-            // and returning the backtrace for, no need to repeat it as the cause.
-            Error::ExternalError(ref err) => err.cause().cause(),
+            Error::ExternalError(ref err) => err.as_fail().cause(),
             _ => None,
         }
     }