Unknown collation: 'utf8mb4_unicode_ci'

If you are receiving this error your web host does not provide a MySQL-version which is capable to run tables with utf8mb4 collation. The WordPress tables were changed to utf8mb4 with Version 4.2 (released on April, 23rd 2015) to support Emojis, but you need MySQL 5.5.3 to use it. 5.5.3. is from March 2010, so it should normally be widely available.