Remove FAIL_FAST environment variable

fail_fast can be set just by specifying rspec --fail-fast. Previously,
our use of the FAIL_FAST was preventing that from working, though.

I've removed this from the README because it
a) isn't important - nobody needs this to get started developing
b) is described in rspec's docs along with other options

api/spec/spec_helper.rb

@@ -63,8 +63,6 @@
 
  config.extend WithModel
 
- config.fail_fast = ENV['FAIL_FAST'] || false
-
  config.before(:each) do
  Rails.cache.clear
  reset_spree_preferences

backend/spec/spec_helper.rb

@@ -120,8 +120,6 @@
 
  config.extend WithModel
 
- config.fail_fast = ENV['FAIL_FAST'] || false
-
  config.example_status_persistence_file_path = "./spec/examples.txt"
 
  config.order = :random

core/spec/spec_helper.rb

@@ -35,8 +35,6 @@
  config.include Spree::TestingSupport::Preferences
  config.extend WithModel
 
- config.fail_fast = ENV['FAIL_FAST'] || false
-
  config.filter_run focus: true
  config.run_all_when_everything_filtered = true
 

frontend/spec/spec_helper.rb

@@ -104,8 +104,6 @@
  config.include Spree::TestingSupport::ControllerRequests, type: :controller
  config.include Spree::TestingSupport::Flash
 
- config.fail_fast = ENV['FAIL_FAST'] || false
-
  config.example_status_persistence_file_path = "./spec/examples.txt"
 
  config.order = :random

sample/spec/spec_helper.rb

@@ -31,8 +31,6 @@
  # instead of true.
  config.use_transactional_fixtures = false
 
- config.fail_fast = ENV['FAIL_FAST'] || false
-
  config.example_status_persistence_file_path = "./spec/examples.txt"
 
  config.order = :random