Remove redundant util requirement

The Transaction class was requiring bitcore.util.js twice, as `util` and
`JSUtil`. It seems `JSUtil` is the standard name in other classes.

lib/transaction/transaction.js

@@ -5,7 +5,6 @@ var $ = require('../util/preconditions');
 var buffer = require('buffer');
 
 var errors = require('../errors');
-var util = require('../util/js');
 var bufferUtil = require('../util/buffer');
 var JSUtil = require('../util/js');
 var BufferReader = require('../encoding/bufferreader');
@@ -47,9 +46,9 @@ function Transaction(serialized) {
   if (serialized) {
     if (serialized instanceof Transaction) {
       return Transaction.shallowCopy(serialized);
-    } else if (util.isHexa(serialized)) {
+    } else if (JSUtil.isHexa(serialized)) {
       this.fromString(serialized);
-    } else if (util.isValidJSON(serialized)) {
+    } else if (JSUtil.isValidJSON(serialized)) {
       this.fromJSON(serialized);
     } else if (bufferUtil.isBuffer(serialized)) {
       this.fromBuffer(serialized);