Specify Bourbon version pessimistically

This matches how we specify the Bourbon dependency in Bitters and means
we won't have to coordinate releases between the two so closely.

Fixes #805

lib/suspenders/generators/stylesheet_base_generator.rb

@@ -7,7 +7,7 @@ class StylesheetBaseGenerator < Rails::Generators::Base
       File.dirname(__FILE__))
 
     def add_stylesheet_gems
-      gem "bourbon", "5.0.0.beta.7"
+      gem "bourbon", "~> 5.0.0.beta.7"
       gem "neat", "~> 2.0.0.beta.1"
       gem "refills", group: [:development, :test]
       Bundler.with_clean_env { run "bundle install" }