Please support older versions of Rust about error-chain

[joshtriplett]

Currently working on a patch for this.

[Yamakaky]

Cool!

[joshtriplett]

Mostly there; ran into what seems like a bug in #[derive(Debug)] in Rust 1.10. Trying to debug Debug.

See https://is.gd/yDJ60j for a test case.

[joshtriplett]

I think I ran into rust-lang/rust#22250 , fixed in Rust 1.11.

[joshtriplett]

#103 fixes this enough for some use cases. I don't see any obvious way to get backtrace working without statement blocks, and I don't want to add a pile of conditionals to make a subset of the tests work (avoiding all tests that use #[cfg(...)] on variants within error_chain!), but the crate itself works perfectly with #103.

I submitted the work in progress to make the tests and examples work as #104 for reference, just so that work doesn't get lost if anyone else wants to try it.

[joshtriplett]

Thanks for all the quick responses on #103.

[Yamakaky]

0.7.2 released.