Specify compatible rails versions in gemspec (#163)

blazer.gemspec

@@ -17,7 +17,7 @@ Gem::Specification.new do |spec|
   spec.test_files    = spec.files.grep(%r{^(test|spec|features)/})
   spec.require_paths = ["lib"]
 
-  spec.add_dependency "rails"
+  spec.add_dependency "rails", ">= 4"
   spec.add_dependency "chartkick"
   spec.add_dependency "safely_block", ">= 0.1.1"