Update packaging files

  * d/control (Standards-Version): Bump to 4.6.0.
    (Build-Depends): Bump to ruby-activerecord >= 2:6.1 (closes: #996119).
    (Depends): Add ${ruby:Depends}.
  * d/copyright: Add Upstream-Contact field.
    (Copyright): Add team.
  * d/rules: Use gem installation layout and install changelog.
  * d/upstream/metadata: Add Archive and Changelog fields.