Don't limit sign-in attempts on test environment

Without this modification, the selenium tests often trigger a 429 / Too Many Requests error. (Perhaps this should be applied to "/users/password" as well?)

config/initializers/rack_attack.rb

@@ -21,5 +21,6 @@
 
 # Throttle attempts to a particular path. 4 POSTs to /users/sign_in every 30 seconds
 Rack::Attack.throttle "logins/ip", limit: 4, period: 30.seconds do |req|
-  req.post? && req.path == "/users/sign_in" && req.ip
+  # Don't apply sign-in rate-limiting to test environment 
+  req.post? && req.path == "/users/sign_in" && req.ip unless Rails.env.test?
 end