add test for Jekyll version

spec/jekyll_seo_tag_integration_spec.rb

@@ -23,6 +23,11 @@ RSpec.describe Jekyll::SeoTag do
     expect(output).to match(%r!Jekyll SEO tag v#{version}!i)
   end
 
+  it "outputs jekyll version" do
+    jekyll_version = Jekyll::VERSION
+    expect(output).to match(%r!Jekyll v#{jekyll_version}!i)
+  end
+
   it "outputs valid HTML" do
     site.process
     options = {
@@ -296,6 +301,7 @@ RSpec.describe Jekyll::SeoTag do
         expected = <<-EOS
 <!-- Begin Jekyll SEO tag v#{version} -->
 <title>Foo</title>
+<meta name="generator" content="Jekyll v3.5.2" />
 <meta property="og:title" content="Foo" />
 <meta property="og:locale" content="en_US" />
 <link rel="canonical" href="http://example.invalid/page.html" />