Alter license in Cargo.toml to match recommended format

[wezm]

The Cargo docs recommend using / when multiple licenses are present. The convention seems to be Apache-2.0/MIT for dual licenced crates. For example running cargo licence on a project of mine shows 59 results with this format. Using this more prevalent format means failure doesn't stand out when evaluating licenses with cargo license or similar.

If you're ok with this change I can do the same to failure_derive.

[withoutboats]

See #117. The cargo guidance is being changed, the license format in failure is preferred.

[withoutboats]

cargo docs PR about this: rust-lang/cargo#4898

[wezm]

Good to know. I'll change my own crates to use the new format :-)