5.2.0: Update goss from 0.3.16 to 0.3.21, add support to update /etc/apt/sources.list, remove deprecated bases

Added

• Utility script to update Ubuntu's /etc/apt/sources.list [#76]
  • shunit2 library and tests for the utility script
  • Documentation on how to use the utility script

Changed

• Updated goss from 0.3.16 to 0.3.21 [#78]

Removed

• Stop building deprecated bases: alpine, centos-7, ubuntu-18.04 [#79]