Bump html-pipeline to 2.8.4

In 2.8.3, it was possible to modify a constant without knowing it, which
led to a major performance regression (https://gitlab.com/gitlab-org/gitlab-ce/issues/49409).
Now attempting to modify the HTML::Pipeline::SanitizationFilter.WHITELIST will result in
"can't modify frozen Hash".

See https://github.com/jch/html-pipeline/pull/299.

Gemfile.lock

@@ -397,7 +397,7 @@ GEM
     hipchat (1.5.2)
       httparty
       mimemagic
-    html-pipeline (2.8.3)
+    html-pipeline (2.8.4)
       activesupport (>= 2)
       nokogiri (>= 1.4)
     html2text (0.2.0)