Update dependencies to enable Greenkeeper 🌴 (#8)

* chore(package): update dependencies

* docs(readme): add Greenkeeper badge

README.md

@@ -1,5 +1,7 @@
 # Occurrences  [![Twitter Follow](https://img.shields.io/twitter/follow/proustibat.svg?style=social&label=Follow)](https://twitter.com/proustibat) [![GitHub top language](https://img.shields.io/github/languages/top/proustibat/occurences.svg)](https://github.com/proustibat/occurences) [![GitHub package version](https://img.shields.io/github/package-json/v/proustibat/occurences.svg)]() [![npm](https://img.shields.io/npm/v/occurences.svg)](https://www.npmjs.com/package/occurences)
 
+[![Greenkeeper badge](https://badges.greenkeeper.io/proustibat/occurences.svg)](https://greenkeeper.io/)
+
 
 Calculate the number of occurrences of each word in a text.
 Words smaller than two letters will be ignored.

package.json

@@ -13,12 +13,12 @@
     "words"
   ],
   "devDependencies": {
-    "chai": "^3.5.0",
+    "chai": "^4.1.2",
     "commitizen": "^2.9.6",
-    "coveralls": "^2.13.3",
+    "coveralls": "^3.0.0",
     "cz-conventional-changelog": "^2.0.0",
     "istanbul": "^0.4.5",
-    "mocha": "^3.5.3"
+    "mocha": "^5.0.0"
   },
   "scripts": {
     "test": "mocha --reporter spec",