changes to backtrace crate

[AndyGauge]

https://travis-ci.com/rust-lang-nursery/error-chain/jobs/205173078
|
100 | Symbol::Dladdr(ref s) => return s.addr(),
| ^^^^^^^^ expected enum libc::c_void, found enum std::os::raw::c_void
|
= note: expected type core::option::Option<*mut libc::c_void>
found type core::option::Option<*mut std::os::raw::c_void>
error[E0308]: mismatched types

[palfrey]

I think this is fixed now by the minimum version bumps?