travis: allow test failures on MySQL 5.7

<pre>
ActiveRecord::StatementInvalid: Mysql2::Error:
  Table 'performance_schema.session_variables' doesn't exist:
  SHOW VARIABLES LIKE 'character_set_database'
</pre>

Refer:

* rails/rails#21108
* rails/rails#21318

git-svn-id: http://svn.redmine.org/redmine/trunk@14525 e93f8b46-1217-0410-a6f0-8f06a7374b81

.travis.yml

@@ -47,6 +47,9 @@ matrix:
     - env: "SUITE=ui          DB=mariadb-5.5"
     - env: "SUITE=ui          DB=mariadb-10.0"
     - env: "SUITE=ui          DB=sqlite3"
+    - env: "SUITE=units       DB=mysql-5.7-dmr"
+    - env: "SUITE=functionals DB=mysql-5.7-dmr"
+    - env: "SUITE=integration DB=mysql-5.7-dmr"
     # SCM tests fail randomly due to IO.popen().
     # http://www.redmine.org/issues/19091
     # https://github.com/jruby/jruby/issues/779