set default schema dump format as sql

we use lots of db specific stuff; don't bother trying to dump to ruby
(it also takes forever)

Change-Id: I1ccacc164ffed10c5430c8361c8363761c02ce38
Reviewed-on: https://gerrit.instructure.com/37744
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>