added test for default configuration

.travis.yml

@@ -7,6 +7,7 @@ services:
   - docker
 
 env:
+  - DB_TYPE=default
   - DB_TYPE=sqlite
   - DB_TYPE=mariadb
   - DB_TYPE=postgresql

tests/docker-compose.default.yml

@@ -0,0 +1,9 @@
+version: '2'
+services:
+  wallabag:
+    build:
+      context: ../
+    image: wallabag:default
+    container_name: wallabag
+    ports:
+      - "127.0.0.1:80:80"