use github directive for letter_opener gem

so that it will use https

Change-Id: I6414d8ee064a98d6726ce5633ec1a960c3804b2e
Reviewed-on: https://gerrit.instructure.com/36722
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Rob Orton <rob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>