change to Bignum 6.2 and fix grunt problem

Gruntfile.js

@@ -39,7 +39,7 @@ module.exports = function(grunt) {
         tasks: ['markdown']
       },
       scripts: {
-        files: ['**/*.js', '**/*.html', '!**/node_modules/**', '!browser/bundle.js', '!browser/load_test_data.js'],
+        files: ['**/*.js', '**/*.html', '!**/node_modules/**', '!browser/bundle.js', '!browser/testdata.js'],
         tasks: ['browserify' /*, 'mochaTest'*/ ],
       },
     },

package.json

@@ -34,7 +34,7 @@
     "base58-native": "=0.1.3",
     "bindings": "=1.1.1",
     "bufferput": "git://github.com/bitpay/node-bufferput.git",
-    "bignum": "=0.6.1",
+    "bignum": "=0.6.2",
     "binary": "=0.3.0",
     "step": "=0.0.4",
     "buffers": "=0.1.1",