packages ubuntu: fix archive download error on trusty
packages ubuntu: fix archive download error on trusty

MySQL 5.6.33 is provided by universe repository.

TODO: need more fixes to build correctly.

ref. https://launchpadlibrarian.net/291155545/buildlog_ubuntu-trusty-amd64.mroonga-5.6_6.10-2~trusty3_BUILDING.txt.gz

mroonga/mroonga,mroonga/mroonga,mroonga/mroonga,mroonga/mroonga