fix README config/application.rb for rails5

[tabio]

The following error is displayed during rails5 upgrade.

`assert_index': No such middleware to insert before: ActionDispatch::ParamsParser (RuntimeError)

There is no ActionDispatch::ParamsParser at rails5 middleware list.

# rails5
bin/rails middleware
use Rack::Sendfile
use ActionDispatch::Static
use ActionDispatch::Executor
use ActiveSupport::Cache::Strategy::LocalCache::Middleware
use Rack::Runtime
use Rack::MethodOverride
use ActionDispatch::RequestId
use RequestStore::Middleware
use Sprockets::Rails::QuietAssets
use Rails::Rack::Logger
use ActionDispatch::ShowExceptions
use ActionDispatch::DebugExceptions
use ActionDispatch::RemoteIp
use ActionDispatch::Reloader
use ActionDispatch::Callbacks
use ActiveRecord::Migration::CheckPending
use ActionDispatch::Cookies
use ActionDispatch::Session::RedisStore
use ActionDispatch::Flash
use Rack::Head
use Rack::UserAgent
use Rack::ConditionalGet
use Rack::ETag
use ExceptionNotification::Rack
use Bullet::Rack

So, I upgraded README.md for rails5.

[coveralls]

Coverage decreased (-0.02%) to 99.184% when pulling 2082861 on tabio:fix_readme_for_rails5 into ce30972 on dtaniwaki:master.