tell browsers they can cache can assets for a year

test plan:
* first, clear out all the cdn assets on your test
s3 bucket by running this in rails console:
Canvas::CDN::S3Uploader.new.bucket.objects.with_prefix('dist').delete_all
* compile_assets
* run bundle exec rake canvas:cdn:upload_to_s3
* access canvas in your browser
* on js/css/image assets (like common-xxxxx.js)
  you should see a "max age 1 year" header

Change-Id: I7847e614a0e3066686bda32f8854263f78cc168f
Reviewed-on: https://gerrit.instructure.com/59184
Tested-by: Jenkins
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Jeremy Putnam <jeremyp@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>

lib/canvas/cdn/s3_uploader.rb

@@ -29,8 +29,8 @@ module Canvas
         end
       end
 
-      def fingerprint?(path)
-        /-[0-9a-fA-F]{32}$/.match(path.basename(path.extname).to_s)
+      def fingerprinted?(path)
+        /-[0-9a-fA-F]{10}$/.match(path.basename(path.extname).to_s)
       end
 
       def font?(path)
@@ -43,10 +43,9 @@ module Canvas
 
       def options_for(path)
         options = {acl: :public_read, content_type: mime_for(path)}
-        if fingerprint?(path)
+        if fingerprinted?(path)
           options.merge!({
-            cache_control: "public, max-age=#{1.year}",
-            expires: 1.year.from_now.httpdate
+            cache_control: "public, max-age=#{1.year}"
           })
         end