Allow plugins to specify a minimum requires version

spec/components/plugin/metadata_spec.rb

@@ -10,6 +10,7 @@ describe Plugin::Metadata do
 # version: 0.1
 # authors: Frank Zappa
 # url: http://discourse.org
+# required version: 1.3.0beta6+48
 
 some_ruby
 TEXT
@@ -19,6 +20,7 @@ TEXT
       expect(metadata.version).to eq("0.1")
       expect(metadata.authors).to eq("Frank Zappa")
       expect(metadata.url).to eq("http://discourse.org")
+      expect(metadata.required_version).to eq("1.3.0beta6+48")
     end
   end