build: support MariaDB 5.5

Author: dougwilson

.travis.yml

@@ -16,6 +16,8 @@ env:
   - "DOCKER_MYSQL_TYPE=mysql DOCKER_MYSQL_VERSION=5.5"
 matrix:
   include:
+    - node_js: "6.10"
+      env: "DOCKER_MYSQL_TYPE=mariadb DOCKER_MYSQL_VERSION=5.5"
     - node_js: "6.10"
       env: "DOCKER_MYSQL_TYPE=mariadb DOCKER_MYSQL_VERSION=10.0"
     - node_js: "6.10"