add RUST_BACKTRACE=1 to appveyor build
add RUST_BACKTRACE=1 to appveyor build

aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust