Remove deprecation warning from download_adapter_spec
The proxy doesn't support user/password configuration when downloading
BOSH stemcell

ISSUE：http://rnd-github.huawei.com/paas/bosh/issues/36
An error occurred while running the follow command.
 CANDIDATE_BUILD_NUMBER=2624 bundle exec rake stemcell:build_with_local_os_image[openstack,ubuntu,trusty,ruby,/tmp/ubuntu_base_image_trusty.tgz]
Error dispaly:
I, [2014-07-09T13:37:29.622936 #9994] INFO -- : CANDIDATE_BUILD_NUMBER is 2624. Using candidate build.
I, [2014-07-09T13:37:29.623381 #9994] INFO -- : Downloading http://bosh-ci-pipeline.s3.amazonaws.com/2624/release/bosh-2624.tgz to tmp/bosh-2624.tgz
rake aborted!
Errno::ENOENT: No such file or directory - tmp/bosh-2624.tgz
/root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:30:in initialize' /root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:30:inopen'
/root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:30:in write_response' /root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:24:inblock (2 levels) in download_file'
/usr/local/rvm/gems/ruby-1.9.3-p545/gems/rest-client-1.6.7/lib/restclient/net_http_ext.rb:51:in request' /root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:22:inblock in download_file'
/root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:21:in download_file' /root/workspaces/bosh/bosh-dev/lib/bosh/dev/download_adapter.rb:12:indownload'
/root/workspaces/bosh/bosh-dev/lib/bosh/dev/build.rb:142:in release_tarball_path' /root/workspaces/bosh/bosh-dev/lib/bosh/dev/tasks/stemcell.rake:96:inblock (2 levels) in '
/usr/local/rvm/gems/ruby-1.9.3-p545/bin/ruby_executable_hooks:15:in eval' /usr/local/rvm/gems/ruby-1.9.3-p545/bin/ruby_executable_hooks:15:in'
Tasks: TOP => stemcell:build_with_local_os_image
(See full trace by running task with --trace)