Remove yarn, add npm

Author: ssurabhi10

package.json

@@ -7,19 +7,18 @@
   "private": false,
   "engines": {
     "node": ">=4.8.0",
-    "npm": ">=2.15.11",
-    "yarn": ">=0.20.3"
+    "npm": ">=2.15.11"
   },
   "scripts": {
     "start": "gulp serve",
-    "start:debug": "cross-env DEBUG=express-mongoose-es6-rest-api:* yarn start",
+    "start:debug": "cross-env DEBUG=express-mongoose-es6-rest-api:* npm start",
     "build": "gulp",
     "lint": "esw *.js server config --color",
-    "lint:watch": "yarn lint -- --watch",
+    "lint:watch": "npm lint -- --watch",
     "test": "cross-env NODE_ENV=test ./node_modules/.bin/mocha --ui bdd --reporter spec --colors --compilers js:babel-core/register server/tests --recursive",
-    "test:watch": "yarn test -- --watch",
+    "test:watch": "npm test -- --watch",
     "test:coverage": "cross-env NODE_ENV=test ./node_modules/.bin/istanbul cover _mocha -- --ui bdd --reporter spec --colors --compilers js:babel-core/register server/tests --recursive",
-    "test:check-coverage": "yarn test:coverage && istanbul check-coverage",
+    "test:check-coverage": "npm test:coverage && istanbul check-coverage",
     "report-coverage": "coveralls < ./coverage/lcov.info"
   },
   "repository": {