add missing node module

package.json

@@ -25,6 +25,7 @@
   "homepage": "https://github.com/oslab-fr/lesspass-npm#readme",
   "devDependencies": {
     "babel-cli": "^6.5.1",
+    "babel-core": "^6.5.2",
     "babel-preset-es2015": "^6.5.0",
     "mocha": "^2.4.5",
     "webpack": "^1.12.13"