Fix broken build due to the package was not build before publish (#17)

Author: chenzhihao

package.json

@@ -1,6 +1,6 @@
 {
   "name": "easy-promise-queue",
-  "version": "0.5.5",
+  "version": "0.5.6",
   "engines": {
     "node": ">=8.0.0"
   },
@@ -9,7 +9,8 @@
   "types": "dist/PromiseQueue.d.ts",
   "scripts": {
     "test": "jest",
-    "build": "tsc --declaration"
+    "build": "tsc --declaration",
+	"prepublishOnly:": "npm build"
   },
   "author": "[email protected]",
   "license": "MIT",