dev: pin rake to ~> 12.3 for Ruby 2.0/2.1 compatibility

Author: dbongo

callable.gemspec

@@ -25,6 +25,6 @@ Gem::Specification.new do |spec|
   spec.require_paths = ["lib"]
 
   spec.add_development_dependency "bundler", ">= 1.17"
-  spec.add_development_dependency "rake", "~> 13.0"
+  spec.add_development_dependency "rake", "~> 12.3"
   spec.add_development_dependency "rspec", "~> 3.0"
 end