spec: add a rake task to execute migration_lint
spec: add a rake task to execute migration_lint

this will allow it the script to be easily chained with other test
related rake tests

test-plan:
* modify a migration to have syntax that will migration_lint
* execute 'bundle exec rake db:migration_lint', should return an error
* revert migration change
* execute 'bundle exec rake db:migration_lint', should not return an
* error

Change-Id: I5b0dc9d27759662141e11bf1f42cdf376b67ad96
Reviewed-on: https://gerrit.instructure.com/19885
Tested-by: Jenkins <d95b56ce41a2e1ac4cecdd398defd7414407cc08@instructure.com>
Reviewed-by: Jake Sorce <c8d99c2f7cd5f432c163abcd422672b9f77550bb@instructure.com>
Product-Review: Jake Sorce <c8d99c2f7cd5f432c163abcd422672b9f77550bb@instructure.com>
QA-Review: Jake Sorce <c8d99c2f7cd5f432c163abcd422672b9f77550bb@instructure.com>

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