Do not import node libs on browser. Fixes web3#923

I'm assuming the problem happened when web3 was imported with require, and
then webpack was used on it. If so, this was probably caused because I was
incorrectly importing Node.js related libs on the browser release. This
fixes it by correctly using the browser field on package.json.

packages/web3-bzz/package.json

@@ -6,7 +6,7 @@
     "license": "LGPL-3.0",
     "main": "src/index.js",
     "dependencies": {
-        "swarm-js": "0.1.30",
+        "swarm-js": "0.1.33",
         "underscore": "1.8.3"
     }
 }