chore(package): update sinon to version 2.3.7

Closes #15

Author: greenkeeper[bot]

package.json

@@ -35,7 +35,7 @@
     "mocha": "^3.0.2",
     "proxyquire": "^1.7.10",
     "rimraf": "^2.5.4",
-    "sinon": "^1.17.6",
+    "sinon": "^2.3.7",
     "sinon-chai": "^2.8.0"
   }
 }