Log Message:
enchant2: update to 2.2.15.

2.2.15 (December 22, 2020)
--------------------------

Specify that nuspell >= 4.1.0 is required.

Fix some space leaks in the tests.

2.2.14 (December 10, 2020)
--------------------------

Improve installation instructions: in particular, add a note about where to
find them to README.

The nuspell backend is updated for newer versions.