Disable invisible captcha spinner

The spinner was added in version 2.0.0, so by disabling it we're keeping
the behavior we've got in existing CONSUL installations.

The problem with the spinner is installations need to add a secret token
and share it across instances (when they've got more than one), and if
they don't, invisible captcha might report false positives.

I guess we could use `Rails.application.secrets.secret_key_base` as
secret token, but since we haven't tested this feature at all, it's
better to disable it for now.

config/initializers/invisible_captcha.rb

@@ -1,5 +1,6 @@
 InvisibleCaptcha.setup do |config|
   config.visual_honeypots    = false
+  config.spinner_enabled     = false
   config.timestamp_threshold = 4.seconds
   # Leave these unset if you want to use I18n (see below)
   # config.error_message           = 'You are a robot!'