Release 2.0.0

[lmakarov]

• Major refactoring and version updates
  • Removed Varnish 3.0 and 5.2 as they are EOL/retired
  • Added Varnish 6.0 and 6.1
  • Using the latest Alpine 3.9 for all currently supported versions (4.1, 6.0, 6.1) and installing varnish as an apk package from Alpine repos
  • Switched to supervisord for process management (running varnishd and varnishcsa)
• Give users the ability to set Varnish secret via VARNISH_SECRET environment variable (#9)
• Added BAN/PURGE support
• Added Docker healthcheck support
• Added automated tests
• Building and testing images with Travis CI
• Updated README and LICENSE year