Remove backtrace dependency.

[briansmith]

Disable the default "backtrace" feature of error-chain to remove the
dependency on "backtrace". The replacement for error-chain, the
failure crate, will disable backtraces by default.

Contributed on behalf of Buoyant, Inc.

Signed-off-by: Brian Smith brian@briansmith.org

[bluejekyll]

Thank you. Looks great.

[codecov]

Codecov Report

Merging #416 into master will increase coverage by 0.14%.
The diff coverage is n/a.

@@            Coverage Diff            @@
##           master    #416      +/-   ##
=========================================
+ Coverage   86.26%   86.4%   +0.14%     
=========================================
  Files         133     133              
  Lines       13405   13405              
=========================================
+ Hits        11564   11583      +19     
+ Misses       1841    1822      -19

Impacted Files	Coverage Δ
proto/src/tcp/tcp_client_stream.rs	92.4% <0%> (-1.27%)	⬇️
rustls/src/tls_client_stream.rs	100% <0%> (+17.64%)	⬆️
rustls/src/tls_stream.rs	82.85% <0%> (+20%)	⬆️
resolver/src/tls/dns_over_rustls.rs	100% <0%> (+100%)	⬆️

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