Add version requirement for activeresource

shopify_api.gemspec

@@ -21,9 +21,9 @@ Gem::Specification.new do |s|
 
   s.rdoc_options = ["--charset=UTF-8"]
   s.summary = %q{ShopifyAPI is a lightweight gem for accessing the Shopify admin REST web services}
-  s.license = 'MIT'
+  s.license = "MIT"
 
-  s.add_dependency("activeresource")
+  s.add_dependency("activeresource", " >= 3.0.0")
   s.add_dependency("rack")
 
   s.add_development_dependency("mocha", ">= 0.9.8")