remove unused dev-dependency

Author: wooorm

package.json

@@ -30,8 +30,7 @@
   ],
   "devDependencies": {
     "mocha": "^3.2.0",
-    "txt-to-ast": "^1.1.0",
-    "unist-builder": "^1.0.1"
+    "txt-to-ast": "^1.1.0"
   },
   "dependencies": {
     "object-assign": "^4.0.1"