Make spring a runtime option for Guard

Guardfile

@@ -2,8 +2,9 @@
 
 # More info at https://github.com/guard/guard#readme
 
-# If using spring, change command to "spring rspec"
-guard :rspec, cmd: "bundle exec rspec" do
+cmd = ENV['SPRING'] ? 'spring rspec' : 'bundle exec rspec'
+
+guard :rspec, cmd: cmd do
   require "guard/rspec/dsl"
   dsl = Guard::RSpec::Dsl.new(self)
 

doc/development/testing_guide/best_practices.md

@@ -50,13 +50,7 @@ Use [guard](https://github.com/guard/guard) to continuously monitor for changes
 bundle exec guard
 ```
 
-When using spring and guard together, consider adjusting Guardfile:
-
-```ruby
-guard :rspec, cmd: "spring rspec" do
-  # ...
-end
-```
+When using spring and guard together, use `SPRING=1 bundle exec guard` instead to make use of spring.
 
 ### General guidelines