Migrate down before each migration unit test
Migrate down before each migration unit test

`migrate!` helper triggers a migration that is under the test, so we
need to revert it before each subsequent example. This means that we
need to place it in `before(:each, :migration)` hook.

We still want to migrate everything up *after* each *context*, so in
otherwords we need to place migrate up helper in `before(:context,
:migration)`.

iiet/iiet-git,axilleas/gitlabhq,iiet/iiet-git,t-zuehlsdorff/gitlabhq,stoplightio/gitlabhq,jirutka/gitlabhq,jirutka/gitlabhq,dreampet/gitlab,axilleas/gitlabhq,axilleas/gitlabhq,mmkassem/gitlabhq,stoplightio/gitlabhq,t-zuehlsdorff/gitlabhq,jirutka/gitlabhq,mmkassem/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,dreampet/gitlab,stoplightio/gitlabhq,mmkassem/gitlabhq,t-zuehlsdorff/gitlabhq,axilleas/gitlabhq,jirutka/gitlabhq,iiet/iiet-git,dreampet/gitlab,iiet/iiet-git,stoplightio/gitlabhq,dreampet/gitlab