Bump minimum Mocha version to >= 1.4.0

This is necessary due using `mocha/minitest` instead of `mocha/setup`.

The minitest require was introduced in Mocha 1.0.0 but was previously
named `mocha/mini_test` (with an underscore). It was renamed in 1.4.0.

shopify_api.gemspec

@@ -31,7 +31,7 @@ Gem::Specification.new do |s|
   s.add_runtime_dependency("rack")
   s.add_runtime_dependency("graphql-client")
 
-  s.add_development_dependency("mocha", ">= 0.9.8")
+  s.add_development_dependency("mocha", ">= 1.4.0")
   s.add_development_dependency("webmock")
   s.add_development_dependency("minitest", ">= 4.0")
   s.add_development_dependency("rake")