Fix oembed_retrieve to allow HTTPS oEmbed LTI endpoints

Net::HTTP.get(uri) does't support HTTPS URIs; you need to create a new Net::HTTP object, set use_ssl to true and read the response object body.
Use Canvas::HTTP instead of raw Net::HTTP
Merge pull request #101 from grahamb/canvas-231-https-oembed-fix

Fix oembed_retrieve to allow HTTPS oEmbed LTI endpoints