update deps. (#19)

Author: gucong3000

package.json

@@ -1,6 +1,6 @@
 {
   "name": "postcss-markdown",
-  "version": "0.27.0",
+  "version": "0.28.0",
   "description": "PostCSS syntax for parsing Markdown",
   "repository": {
     "type": "git",
@@ -42,18 +42,18 @@
   },
   "peerDependencies": {
     "postcss": ">=5.0.0",
-    "postcss-syntax": ">=0.27.0"
+    "postcss-syntax": ">=0.28.0"
   },
   "devDependencies": {
     "chai": "^4.1.2",
     "codecov": "^3.0.2",
     "mocha": "^5.2.0",
     "nyc": "^12.0.2",
     "postcss": "^6.0.22",
-    "postcss-html": ">=0.27.0",
+    "postcss-html": ">=0.28.0",
     "postcss-less": "^2.0.0",
     "postcss-scss": "^1.0.5",
-    "postcss-syntax": ">=0.27.0",
+    "postcss-syntax": ">=0.28.0",
     "sugarss": "^1.0.1"
   }
 }