Small refactoring and improvements, fix travis setup

Description:
- Refactor Pluggable module a bit to fit rubocop constraints.
- Fix `Service::SpecHelpers` module to extend example goups with
it's ClassMethods, add test for `service_context` method.
- As it was before, `require` plugin file when it is used, but
is not registered.
- Refactor `SpecHelper` for common approach.
- Fix `.travis.yml` to specify proper Ruby version

.travis.yml

@@ -1,4 +1,4 @@
 language: ruby
 rvm:
-  - 2.2.0
+  - 2.4.0
 before_install: gem install bundler -v 1.16.0

lib/zen/service/plugins.rb

@@ -3,6 +3,8 @@
 module Zen
   module Service::Plugins
     def self.fetch(name)
+      require("zen/service/plugins/#{name}") unless plugins.key?(name)
+
       plugins[name] || raise("extension `#{name}` is not registered")
     end
 

lib/zen/service/plugins/pluggable.rb

@@ -14,6 +14,10 @@ def use(name, **opts)
           return extension
         end
 
+        use_extension(extension, name, **opts)
+      end
+
+      private def use_extension(extension, name, **opts)
         include extension
         extend extension::ClassMethods if extension.const_defined?(:ClassMethods)
 

lib/zen/service/spec_helpers.rb

@@ -2,6 +2,10 @@
 
 module Zen
   module Service::SpecHelpers
+    def self.included(target)
+      target.extend(ClassMethods)
+    end
+
     # Example:
     #   stub_service(MyService)
     #     .with_atributes(foo: 'foo')

spec/spec_helper.rb

@@ -5,15 +5,17 @@
 require "rspec/its"
 
 module SpecHelper
-  module GroupMethods
-    def def_service(&block)
-      let(:service_class) { Class.new(Zen::Service, &block) }
-    end
+  def self.included(target)
+    target.extend(ClassMethods)
+  end
+
+  def build_service(*args)
+    service_class.new(*args)
   end
 
-  module ExampleMethods
-    def build_service(*args)
-      service_class.new(*args)
+  module ClassMethods
+    def def_service(&block)
+      let(:service_class) { Class.new(Zen::Service, &block) }
     end
   end
 end
@@ -29,8 +31,6 @@ def build_service(*args)
     c.syntax = :expect
   end
 
-  config.extend SpecHelper::GroupMethods
-  config.include SpecHelper::ExampleMethods
-
+  config.include SpecHelper
   config.include Zen::Service::SpecHelpers
 end

spec/zen/service/rspec_helpers_spec.rb

@@ -4,10 +4,12 @@
 
 RSpec.describe "Zen::Service::SpecHelpers" do
   class ::SpecHelpersService < Zen::Service
+    use :context
+
     attributes :foo
 
     def execute!
-      failure!
+      context[:foo]
     end
   end
 
@@ -24,4 +26,13 @@ def execute!
       expect(service.result).to eq(6)
     end
   end
+
+  describe "service_context" do
+    service_context { { foo: 5 } }
+
+    it "allows to set service context" do
+      service = SpecHelpersService.new
+      expect(service.execute.result).to eq(5)
+    end
+  end
 end