fix predeploy migrations hiding other migrations
fix predeploy migrations hiding other migrations

when the db:migrate:predeploy task ran, it mutated the list of available
migrations, preventing other tasks that use the migrations list from
executing correctly

test plan:
 - run migrations against a clean database (no schema):
   bin/rake db:migrate:predeploy db:migrate
 - ensure that 20160310225521 was migrated successfully
   bin/rake db:migrate:status | grep 20160310225521

refs SD-915

Change-Id: I06efb4c1b8a0738f8f6254e55426eca405e30b13
Reviewed-on: https://gerrit.instructure.com/74463
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <c4fe2b1d90ef8f2548c8aeabfa633434316f0305@instructure.com>
Product-Review: Mark Severson <cdf1f90aee9d0d073a2a24141ebc71887f3ba533@instructure.com>
QA-Review: Mark Severson <cdf1f90aee9d0d073a2a24141ebc71887f3ba533@instructure.com>

roxolan/canvas-lms,HotChalk/canvas-lms,djbender/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,roxolan/canvas-lms,grahamb/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,djbender/canvas-lms,HotChalk/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,HotChalk/canvas-lms,venturehive/canvas-lms,fronteerio/canvas-lms,sfu/canvas-lms,grahamb/canvas-lms,djbender/canvas-lms,SwinburneOnline/canvas-lms,HotChalk/canvas-lms,roxolan/canvas-lms,sfu/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,fronteerio/canvas-lms,SwinburneOnline/canvas-lms,venturehive/canvas-lms,venturehive/canvas-lms,sfu/canvas-lms,djbender/canvas-lms,grahamb/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,SwinburneOnline/canvas-lms