Add bundle dependencies to gemspec (thor and sass) to enable 'rake update-mapbox' command

Author: nbumbarger

mapbox-rails.gemspec

@@ -20,4 +20,6 @@ Gem::Specification.new do |spec|
 
   spec.add_development_dependency "bundler", "~> 1.3"
   spec.add_development_dependency "rake"
+  spec.add_development_dependency "thor"
+  spec.add_development_dependency "sass"
 end