Update Bourbon to 5.0.0.beta.7

Use a pessimistic version operator.

Author: tysongach

bitters.gemspec

@@ -7,7 +7,7 @@ Gem::Specification.new do |s|
   s.add_development_dependency "rake"
   s.add_development_dependency "rspec"
   s.add_development_dependency "scss_lint", "~> 0.47"
-  s.add_runtime_dependency "bourbon", ">= 5.0.0.beta.6"
+  s.add_runtime_dependency "bourbon", "~> 5.0.0.beta.7"
   s.add_runtime_dependency "sass", "~> 3.4"
   s.add_runtime_dependency "thor", "~> 0.19"
   s.authors = [