fail! has been renamed to panic!

[frewsxcv]

rust-lang/rust#17894

Documentation will need to change

[mxork]

Already taken care of: https://github.com/japaric/rust-by-example/pull/269.

Just waiting on the merge.

[abonander]

@brson This should have been closed by #280

[frewsxcv]

Thanks for letting me know @cybergeek94