Remove mariadb:10.2 from testing as pt-mysql-summary hangs on it.

Author: arvenil

.travis.yml

@@ -33,7 +33,9 @@ env:
     - MYSQL_IMAGE=mariadb:5.5
     - MYSQL_IMAGE=mariadb:10.0
     - MYSQL_IMAGE=mariadb:10.1
-    - MYSQL_IMAGE=mariadb:10.2
+    # pt-mysql-summary hangs on mariadb:10.2
+    # https://jira.percona.com/browse/PT-1549
+    #- MYSQL_IMAGE=mariadb:10.2
     - MONGODB_IMAGE=mongo:3.2
     - MONGODB_IMAGE=mongo:3.4
     - MONGODB_IMAGE=mongo:3.6