Bug#951349: mecab: Fails to build against ruby2.7

[Lucas Kanashiro]

FYI ruby2.7 is already enabled in unstable (the ruby2.7 transition
already started), so no need to use experimental. Here you can find the
build logs with failures:
https://buildd.debian.org/status/package.php?p=mecab

-- 
Lucas Kanashiro