Add MariaDB 10.5 tests (#17)

It's in beta: https://mariadb.com/kb/en/changes-improvements-in-mariadb-105/

Author: adamchainz

.travis.yml

@@ -86,14 +86,17 @@ env:
 # MariaDB:
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.6 MARIADB_VERSION=10.3
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.6 MARIADB_VERSION=10.4
+- COMPOSE_APP=mariadb PYTHON_VERSION=3.6 MARIADB_VERSION=10.5
 
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.7 MARIADB_VERSION=10.2
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.7 MARIADB_VERSION=10.3
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.7 MARIADB_VERSION=10.4
+- COMPOSE_APP=mariadb PYTHON_VERSION=3.7 MARIADB_VERSION=10.5
 
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.8 MARIADB_VERSION=10.2
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.8 MARIADB_VERSION=10.3
 - COMPOSE_APP=mariadb PYTHON_VERSION=3.8 MARIADB_VERSION=10.4
+- COMPOSE_APP=mariadb PYTHON_VERSION=3.8 MARIADB_VERSION=10.5
 
 # Browsers:
 - COMPOSE_APP=chrome PYTHON_VERSION=3.6