Use `described_class` where appropriate

spec/services/image_for_build_service_spec.rb

 require 'spec_helper'
 describe ImageForBuildService do
-  let(:service) { ImageForBuildService.new }
+  let(:service) { described_class.new }
   let(:project) { FactoryGirl.create(:project) }
   let(:commit) { FactoryGirl.create(:commit, project: project, ref: 'master') }
   let(:build) { FactoryGirl.create(:build, commit: commit) }

spec/services/register_build_service_spec.rb

 require 'spec_helper'
 describe RegisterBuildService do
-  let!(:service) { RegisterBuildService.new }
+  let!(:service) { described_class.new }
   let!(:project) { FactoryGirl.create :project }
   let!(:commit) { FactoryGirl.create :commit, project: project }
   let!(:pending_build) { FactoryGirl.create :build, project: project, commit: commit }

spec/services/web_hook_service_spec.rb

@@ -9,7 +9,7 @@ describe WebHookService do
   describe '#execute' do
     it "should execute successfully" do
       stub_request(:post, hook.url).to_return(status: 200)
-      WebHookService.new.build_end(build).should be_truthy
+      described_class.new.build_end(build).should be_truthy
     end
   end
@@ -31,6 +31,6 @@ describe WebHookService do
   end
   def build_data(build)
-    WebHookService.new.send :build_data, build
+    described_class.new.send :build_data, build
   end
 end