textproc/nuspell: update to 5.1.6

Changes: https://github.com/nuspell/nuspell/releases/tag/v5.1.5
Changes: https://github.com/nuspell/nuspell/releases/tag/v5.1.6
Reported by: GitHub (watch releases)

(cherry picked from commit b2a03fd48c6bf93dae897505c9138b126dbe1089)