Allow requests from Google Image Proxy

which should fix email notifications' avatars rendering as broken
images in GMail.

ref: https://app.fizzy.do/5986089/cards/1740

Gemfile.lock

@@ -18,9 +18,9 @@ GIT
 
 GIT
   remote: https://github.com/basecamp/useragent
-  revision: 746e37ba93cef6399920d18ad88903db92ba4e30
+  revision: b253635dc649394c72b761f09234bfd6dc87516a
   specs:
-    useragent (0.16.10)
+    useragent (0.16.11)
 
 GIT
   remote: https://github.com/basecamp/yabeda-activejob.git

test/controllers/allow_browser_test.rb

@@ -41,4 +41,14 @@ class AllowBrowserTest < ActionDispatch::IntegrationTest
 
     assert_response :not_acceptable
   end
+
+  test "Google Image Proxy is allowed" do
+    sign_in_as :kevin
+
+    get cards_path, headers: {
+      "User-Agent" => "Mozilla/5.0 (Windows NT 5.1; rv:11.0) Gecko Firefox/11.0 (via ggpht.com GoogleImageProxy)"
+    }
+
+    assert_response :success
+  end
 end