spec: silence rspec deprecation warning

full warning:

> rspec-rails 3 will no longer automatically infer an example group's spec type
> from the file location. You can explicitly opt-in to this feature using this
> snippet:
>
> RSpec.configure do |config|
>   config.infer_spec_type_from_file_location!
> end
>
> If you wish to manually label spec types via metadata you can safely ignore
> this warning and continue upgrading to RSpec 3 without addressing it.

Change-Id: I0f0ec385a49d6b37bfb236c82ab1027934476441
Reviewed-on: https://gerrit.instructure.com/39299
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Paul Hinze <paulh@instructure.com>
QA-Review: Paul Hinze <paulh@instructure.com>