Fix Rubocop offenses

Vagrantfile

@@ -10,7 +10,7 @@ Vagrant.configure(2) do |config|
     v.cpus = 4
   end
   config.vm.network 'forwarded_port',
-    host_ip: ENV['LISTEN_ADDRESS'] || '127.0.0.1',
+    host_ip: ENV.fetch('LISTEN_ADDRESS', '127.0.0.1'),
     host: 7000,
     guest: 7000
   config.vm.synced_folder '.', '/home/vagrant/codeocean'

config/environments/development.rb

@@ -76,7 +76,7 @@ Rails.application.configure do
   # Raises helpful error messages.
   config.assets.raise_runtime_errors = true
 
-  BetterErrors::Middleware.allow_ip! ENV['TRUSTED_IP'] if ENV['TRUSTED_IP']
+  BetterErrors::Middleware.allow_ip! ENV.fetch('TRUSTED_IP', nil) if ENV['TRUSTED_IP']
 
   # Use an evented file watcher to asynchronously detect changes in source code,
   # routes, locales, etc. This feature depends on the listen gem.

config/initializers/sentry_javascript.rb

@@ -6,7 +6,7 @@ class SentryJavascript
   end
 
   def self.dsn
-    ENV['SENTRY_JAVASCRIPT_DSN']
+    ENV.fetch('SENTRY_JAVASCRIPT_DSN', nil)
   end
 
   def self.release

config/schedule.rb

@@ -22,7 +22,7 @@
 # Learn more: http://github.com/javan/whenever
 
 set :output, "#{Whenever.path}/log/whenever/whenever_$(date +%Y%m%d%H%M%S).log"
-set :environment, ENV['RAILS_ENV'] if ENV['RAILS_ENV']
+set :environment, ENV.fetch('RAILS_ENV', nil) if ENV['RAILS_ENV']
 
 every 1.day, at: '3:00 am' do
   rake 'detect_exercise_anomalies:with_at_least[10,50]'

spec/support/selenium.rb

@@ -3,7 +3,7 @@
 require 'capybara/rspec'
 require 'selenium/webdriver'
 
-if ENV['HEADLESS_TEST'] == 'true' || ENV['USER'] == 'vagrant'
+if ENV.fetch('HEADLESS_TEST', nil) == 'true' || ENV.fetch('USER', nil) == 'vagrant'
   require 'headless'
 
   headless = Headless.new
@@ -14,7 +14,7 @@ Capybara.register_driver :selenium do |app|
   profile = Selenium::WebDriver::Firefox::Profile.new
   profile['intl.accept_languages'] = 'en'
   options = Selenium::WebDriver::Firefox::Options.new
-  options.headless! if ENV['CI'] == 'true'
+  options.headless! if ENV.fetch('CI', nil) == 'true'
   options.profile = profile
   driver = Capybara::Selenium::Driver.new(app, browser: :firefox, capabilities: options)
   driver.browser.manage.window.resize_to(1280, 960)