Remove constants from v8 reference schema

Issue: #340

lib/validate/parsed.js

@@ -383,6 +383,13 @@ module.exports = function(style, reference) {
     validate['*'] = function() {};
 
     validate('', style, reference.$root);
+    if (reference.$version > 7 && style.constants) {
+        validate.constants('constants', style.constants);
+    }
+
+    errors.sort(function (a, b) {
+      return a.line - b.line;
+    });
 
     return errors;
 };

reference/v8.json

@@ -56,23 +56,13 @@
       "type": "transition",
       "doc": "A global transition definition to use as a default across properties."
     },
-    "constants": {
-      "type": "constants",
-      "doc": "An object of constants to be referenced in layers."
-    },
     "layers": {
       "required": true,
       "type": "array",
       "value": "layer",
       "doc": "Layers will be drawn in the order of this array."
     }
   },
-  "constants": {
-    "*": {
-      "type": "*",
-      "doc": "A constant that will be replaced verbatim in the referencing place. This can be anything, including objects and arrays. All variable names must be prefixed with an `@` symbol."
-    }
-  },
   "sources": {
     "*": {
       "type": "source",