Update version in install instructions

Author: existentialmutt

README.md

@@ -11,7 +11,7 @@ Add `rspec-rails` to **both** the `:development` and `:test` groups in the
 
 ```ruby
 group :development, :test do
-  gem 'rspec-rails', '~> 3.7'
+  gem 'rspec-rails', '~> 3.8'
 end
 ```