travis: use master of Groonga even if Mroonga is not bundled case
travis: use master of Groonga even if Mroonga is not bundled case

master is needed for: https://github.com/mroonga/mroonga/commit/6fb253a29522e7e745297a0e0082d9e7e072f962

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