fixed yaml-encoding of licenseSpec

Author: stschiff

src/Poseidon/Package.hs

@@ -204,7 +204,7 @@ instance ToJSON PoseidonYamlStruct where
 instance FromJSON LicenseSpec where
     parseJSON = withObject "LicenseSpec" $ \v -> LicenseSpec
         <$> v .:  "name"
-        <*> v .:  "url"
+        <*> v .:? "url"
         <*> v .:? "file"
 
 instance ToJSON LicenseSpec where