I prefer to simply configure nginx to serve .well-known as static content.

app/assets/javascripts/pages.coffee

@@ -1,3 +0,0 @@
-# Place all the behaviors and hooks related to the matching controller here.
-# All this logic will automatically be available in application.js.
-# You can use CoffeeScript in this file: http://coffeescript.org/

app/assets/stylesheets/pages.scss

@@ -1,3 +0,0 @@
-// Place all the styles related to the Pages controller here.
-// They will automatically be included in application.css.
-// You can use Sass (SCSS) here: http://sass-lang.com/

app/controllers/pages_controller.rb

@@ -1,6 +0,0 @@
-class PagesController < ApplicationController
-  def letsencrypt
-		render text: "key goes here..."
-  end
-end
-# vim: set ts=2 sw=2:

app/helpers/pages_helper.rb

@@ -1,2 +0,0 @@
-module PagesHelper
-end

app/views/pages/letsencrypt.html.erb

@@ -1,2 +0,0 @@
-<h1>Pages#letsencrypt</h1>
-<p>Find me in app/views/pages/letsencrypt.html.erb</p>

config/routes.rb

@@ -1,6 +1,4 @@
 Rails.application.routes.draw do
-  get 'pages/letsencrypt'
-
   get 'welcome/index'
 
   # The priority is based upon order of creation: first created -> highest priority.
@@ -9,10 +7,6 @@ Rails.application.routes.draw do
   # You can have the root of your site routed with "root"
   root 'welcome#index'
 
-  # Static pages controller
-  # controller for letsencrypt
-  get '/.well-known/acme-challenge/:id' => 'pages#letsencrypt'
-
   # Example of regular route:
   #   get 'products/:id' => 'catalog#view'
 

test/controllers/pages_controller_test.rb

@@ -1,9 +0,0 @@
-require 'test_helper'
-
-class PagesControllerTest < ActionController::TestCase
-  test "should get letsencrypt" do
-    get :letsencrypt
-    assert_response :success
-  end
-
-end