Fix warning for lowercase static const

The constant 'word_limits' in example 5 generated a warning due to warn(non_upper_case_globals), which is on by default.
Update usages of fail! to panic!

The fail! macro was renamed to panic! in the Oct 30th, 2014 nightly. See commit: https://github.com/rust-lang/rust/commit/7828c3dd2858d8f3a0448484d8093e22719dbda0
Merge pull request #47 from pmsanford/master

Fix warning for lowercase static const