changed >= to ^ for mkdirp dependency

Allows it to build on Node.js 8

Fix: #66
PR-URL: #67
Credit: @pradeepggowda
Close: #67
Reviewed-by: @isaacs

Author: pradeepggowda

package.json

@@ -14,7 +14,7 @@
   "dependencies": {
     "graceful-fs": "^4.1.2",
     "inherits": "~2.0.0",
-    "mkdirp": ">=0.5 0",
+    "mkdirp": "^0.5 0",
     "rimraf": "2"
   },
   "devDependencies": {