Updating vucoin dependency to 0.20.0

package.json

@@ -36,7 +36,7 @@
     "merkle": "~0.1.0",
     "connect-pgp": "~0.3.0",
     "underscore": "~1.5.2",
-    "vucoin": "~0.19.0",
+    "vucoin": "~0.20.0",
     "request": "~2.31.0",
     "log4js": "~0.6.9",
     "moment": "~2.6.0",