Use double quotes in config/

config/environments/production.rb

@@ -22,7 +22,7 @@ Rails.application.configure do
 
   # Disable serving static files from the `/public` folder by default since
   # Apache or NGINX already handles this.
-  config.serve_static_files = ENV['RAILS_SERVE_STATIC_FILES'].present?
+  config.serve_static_files = ENV["RAILS_SERVE_STATIC_FILES"].present?
 
   # Compress JavaScripts and CSS.
   config.assets.js_compressor = :uglifier
@@ -38,8 +38,8 @@ Rails.application.configure do
   # `config.assets.precompile` and `config.assets.version` have moved to config/initializers/assets.rb
 
   # Specifies the header that your server uses for sending files.
-  # config.action_dispatch.x_sendfile_header = 'X-Sendfile' # for Apache
-  # config.action_dispatch.x_sendfile_header = 'X-Accel-Redirect' # for NGINX
+  # config.action_dispatch.x_sendfile_header = "X-Sendfile" # for Apache
+  # config.action_dispatch.x_sendfile_header = "X-Accel-Redirect" # for NGINX
 
   # Force all access to the app over SSL, use Strict-Transport-Security, and use secure cookies.
   config.force_ssl = true
@@ -58,7 +58,7 @@ Rails.application.configure do
   config.cache_store = :dalli_store, { value_max_bytes: 2000000 }
 
   # Enable serving of images, stylesheets, and JavaScripts from an asset server.
-  # config.action_controller.asset_host = 'http://assets.example.com'
+  # config.action_controller.asset_host = "http://assets.example.com"
 
   # Ignore bad email addresses and do not raise email delivery errors.
   # Set this to true and configure the email server for immediate delivery to raise delivery errors.
@@ -70,12 +70,12 @@ Rails.application.configure do
   # Uncomment the following block of code and add your SMTP service credentials
   # config.action_mailer.delivery_method = :smtp
   # config.action_mailer.smtp_settings = {
-  #   address:              'smtp.example.com',
+  #   address:              "smtp.example.com",
   #   port:                 587,
-  #   domain:               'example.com',
-  #   user_name:            '<username>',
-  #   password:             '<password>',
-  #   authentication:       'plain',
+  #   domain:               "example.com",
+  #   user_name:            "<username>",
+  #   password:             "<password>",
+  #   authentication:       "plain",
   #   enable_starttls_auto: true }
 
   # Enable locale fallbacks for I18n (makes lookups for any locale fall back to