Alias health_check so it responds to health-check

The AWS health check endpoint has been configured to look for the route
`health-check` instead of `health_check`.

Rather than asking them to reconfigure it, we will accept that both
versions (with dash and with underscore) are common, and provide an
alias for the route, so that it responds to both requests.

config/routes.rb

@@ -21,6 +21,7 @@
 
   scope module: "public" do
     get "health_check" => "base#health_check"
+    match "health-check" => "base#health_check", via: :get
     root to: "visitors#index"
   end
 

spec/requests/health_check_spec.rb

@@ -18,6 +18,11 @@
     end
   end
 
+  it "responds to health-check" do
+    get "/health-check", headers: {"CONTENT_TYPE" => "application/json", "ACCEPT" => "application/json"}
+    expect(response).to have_http_status(:ok)
+  end
+
   context "when enqueued is 20" do
     it "returns the correct size" do
       stats_double = double(Sidekiq::Stats, enqueued: 20, retry_size: 0)