Require 'text/html' content_type in append_paloma hook

This hook was responsible for causing DoubleRenderErrors in our app,
specifically within API responses. Our API deals strictly in JSON
(content_type 'application/json'), so we don't want Paloma running there
anyways. Our quick fix was to put

  skip_after_filter :append_paloma_hook

in our API controller superclass, but this is a burden and non-obvious
to other developers. This should give us the behavior we expect.