update Debian files

Fixes the following Lintian tags:
```
W: mintwelcome source: debhelper-but-no-misc-depends mintwelcome
W: mintwelcome source: debian-rules-missing-recommended-target build-indep
W: mintwelcome source: debian-rules-missing-recommended-target build-arch
W: mintwelcome source: binary-arch-rules-but-pkg-is-arch-indep
W: mintwelcome source: ancient-standards-version 3.7.2 (current is 3.9.5)
W: mintwelcome: debian-changelog-line-too-long line 4
E: mintwelcome: copyright-should-refer-to-common-license-file-for-gpl
W: mintwelcome: copyright-without-copyright-notice
```