travis: add ignore list to MariaDB 10.1.6

Because MariaDB 10.1.6 is built with newer GCC rather than system
GCC. It breaks build with system GCC.

We can remove MariaDB 10.1.6 from ignore list after Travis CI upgrades
its Ubuntu version.

.travis.yml

@@ -21,7 +21,7 @@ matrix:
 #    - env: MYSQL_VERSION=mariadb-10.0.20 CFLAGS=-funsigned-char CXXFLAGS=-funsigned-char
     - env: MYSQL_VERSION=mariadb-10.0.20 MROONGA_BUNDLED=yes
 #    - env: MYSQL_VERSION=mariadb-10.0.20 MROONGA_BUNDLED=yes MROONGA_TEST_EMBEDDED=yes
-#    - env: MYSQL_VERSION=mariadb-10.1.6
+    - env: MYSQL_VERSION=mariadb-10.1.6
 install:
   - tools/travis/install.sh
 before_script: