Release 2.0.0

[lmakarov]

• Switched to official Apache Solr images as the base
• Dropped Solr 3 and Solr 4 (EOL)
  • The latest available images for Docksal are docksal/1.0-solr3 and docksal/1.0-solr4 respectively
• Added Solr 5.5, 6.6 and 7.5
• Drupal config sets support for Search API Solr and ApacheSolr modules
• Refactored build process
• Updated README and LICENSE year

Closes #1, closes docksal/docksal#733