Update to hapi 5.x

Per [hapi 5.0 migration checklist](https://github.com/spumko/hapi/issues/1645), it doesn't look like there is any breaking change; simply updating `peerDependencies` and `devDependencies` should be enough.

All tests pass, but code coverage still isn't 100% (unaffected by hapi update):

```
34 tests complete (423 ms)

No global variable leaks detected.

Coverage: 91.51%

lib/index.js missing coverage on line(s): 71, 106, 119, 141, 143, 165, 190, 204, 205, 207, 208, 237, 238, 248, 249, 250, 259, 262, 263, 277, 291, 306, 314, 327, 328, 338, 342, 357, 469, 489, 500, 513
Code coverage below threshold: 91.51 < 100

make: *** [test-cov] Error 1
npm ERR! Test failed.  See above for more details.
npm ERR! not ok code 0
```