chore(package): update time-span to version 3.1.0

Closes #17

Author: greenkeeper[bot]

package.json

@@ -39,7 +39,7 @@
     "minimist": "^1.2.0",
     "nyc": "^11.0.2",
     "pretty-bytes": "^5.0.0",
-    "time-span": "^2.0.0",
+    "time-span": "^3.1.0",
     "xo": "^0.19.0"
   }
 }