Update packaging files and fix FTBFS

  * d/compat: Remove obsolete file.
  * d/control: Add Rules-Requires-Root field.
    (Build-Depends): Use debhelper-compat version 12. Remove unrelevant
    version information. Depend on ruby-rspec to fix FTBFS (closes: #952034).
    (Standards-Version): Bump to 4.5.0.
    (Depends): Use ${ruby:Versions} and remove interpreter and libraries.
  * d/ruby-tests.rake: Make it a file instead of a symlink and run tests.