Fix some issues reported by lintian

See merge request !1

Fixes lintian: public-upstream-key-not-minimal
See https://lintian.debian.org/tags/public-upstream-key-not-minimal.html for more details.

Fixes lintian: out-of-date-standards-version
See https://lintian.debian.org/tags/out-of-date-standards-version.html for more details.

Update to upstream version '5.7.0'
with Debian dir 7d92dbe1df76e3e8761f2088c45a0b42e745743d

"debian/master" creates confusion with both the debian/<version>
tag format and the <remote>/<branch> remote branch format used by
Git. Therefore, rename the branch to the more sane name "master".

Upload to unstable, disable patches, add build-deps to debian control, thanks Rene Engelhard for the hint!

configure with --with-hunspell-dictdir=/usr/share/hunspell since that's where the dictionaries are since ages

Update to upstream version '5.5.1'
with Debian dir 6719dd782e679098e903523875a6e53a13b767c9