allow `haskell-src-exts-1.15` in test suite

Closes #22.
(cherry picked from commit 220e91fb0aeb1ddf3dd259e9de34a7cb9e9d1cfc)
Bump version to 0.2.2.7
Release notes for 0.2.2.7
(cherry picked from commit 662188a87acef6e04ecc48cfb788c07c4e3672df)