Fix webpack require failure due to "./" notation

Author: Chris Clark

src/core/server.js

@@ -6,7 +6,7 @@ const url = require('url');
 const HttpsProxyAgent = require('https-proxy-agent');
 const LRU = require('lru-cache');
 const EventEmitter = require('events').EventEmitter;
-const RippleError = require('./').RippleError;
+const RippleError = require('./rippleerror').RippleError;
 const Amount = require('./amount').Amount;
 const RangeSet = require('./rangeset').RangeSet;
 const log = require('./log').internal.sub('server');