travis: allow test failure with system MySQL

https://travis-ci.org/mroonga/mroonga/jobs/18175307 reports:

    query 'INSTALL PLUGIN mroonga SONAME $ha_mroonga_so' failed:
      1126: Can't open shared library 'ha_mroonga.so'
        (errno: 0 API version for STORAGE ENGINE plugin is too different)

But I don't know why...