update ibrowse to 4.0.2

Riak CS also uses ibrowse 4.0.2 via [erlcloud](https://github.com/basho/erlcloud) for testing. When [trying to update webmachine](https://github.com/basho/riak_cs/pull/890) it consequently depends on ibrowse 4.0.1, which conflicts against 4.0.2. This pull request updates dependency to 4.0.2, and relieves version requirement as well.