fail -> panic

Sorry I broke your code with rust-lang/rust#17894 ! Here's a fix :heart:

This also needs to wait on these PRs:

https://github.com/rust-lang/glob/pull/7

https://github.com/alexcrichton/git2-rs/pull/18

https://github.com/alexcrichton/flate2-rs/pull/3

https://github.com/alexcrichton/curl-rust/pull/4

https://github.com/rust-lang/semver/pull/14

https://github.com/servo/rust-url/pull/44

https://github.com/carllerche/hamcrest-rust/pull/7

https://github.com/alexcrichton/toml-rs/pull/24

https://github.com/docopt/docopt.rs/pull/33

https://github.com/lifthrasiir/rust-encoding/pull/53