Reverting to deprecated reference to site:

`site:` is deprecated in favor of `source 'https://api.berkshelf.com'`, however, there's some issue with installing Berkshelf (vaguely referenced here: https://github.com/berkshelf/berkshelf/issues/1261) when doing this:

```sh
==> default: 
==> default: ================================================================================
==> default: Error executing action `install` on resource 'rbenv_gem[2.1.2::berkshelf] (system)'
==> default: ================================================================================
==> default: 
==> default: 
==> default: Mixlib::ShellOut::CommandTimeout
==> default: --------------------------------
==> default: Command timed out after 600s:
==> default: Command execeded allowed execution time, process terminated
==> default: ---- Begin output of export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" ----
==> default: STDOUT: Successfully installed addressable-2.3.6

...

==> default: Building native extensions.  This could take a while...
==> default: STDERR: 
==> default: ---- End output of export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" ----
==> default: Ran export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" returned 
==> default: 
==> default: 
==> default: Cookbook Trace:
==> default: ---------------
==> default: /tmp/vagrant-chef-3/chef-solo-1/cookbooks/rbenv/libraries/chef_rbenv_mixin.rb:30:in `shell_out!'
==> default: /tmp/vagrant-chef-3/chef-solo-1/cookbooks/rbenv/libraries/chef_provider_package_rbenvrubygems.rb:66:in `install_package'
==> default: 
==> default: 
==> default: Resource Declaration:
==> default: ---------------------
==> default: # In /tmp/vagrant-chef-3/chef-solo-1/cookbooks/rbenv/recipes/system.rb
==> default: 
==> default:  46:     rbenv_gem gem['name'] do
==> default:  47:       rbenv_version rubie
==> default:  48: 
==> default:  49:       %w{version action options source}.each do |attr|
==> default:  50:         send(attr, gem[attr]) if gem[attr]
==> default:  51:       end
==> default:  52:     end
==> default:  53:   end
==> default: 
==> default: 
==> default: 
==> default: Compiled Resource:
==> default: ------------------
==> default: # Declared in /tmp/vagrant-chef-3/chef-solo-1/cookbooks/rbenv/recipes/system.rb:46:in `block (2 levels) in from_file'
==> default: 
==> default: rbenv_gem("berkshelf") do
==> default:   provider Chef::Provider::Package::RbenvRubygems
==> default:   action :install
==> default:   retries 0
==> default:   retry_delay 2
==> default:   guard_interpreter :default
==> default:   cookbook_name :rbenv
==> default:   recipe_name "system"
==> default:   rbenv_version "2.1.2"
==> default:   version "3.1.4"
==> default:   package_name "berkshelf"
==> default:   gem_binary "export RBENV_ROOT=\"/usr/local/rbenv\" && export PATH=\"$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH\" && export RBENV_VERSION=\"2.1.2\" && $RBENV_ROOT/shims/gem"
==> default: end
==> default: 
==> default: 
==> default: 
==> default: [2014-07-23T13:49:24+00:00] INFO: Running queued delayed notifications before re-raising exception
==> default: [2014-07-23T13:49:24+00:00] ERROR: Running exception handlers
==> default: [2014-07-23T13:49:24+00:00] ERROR: Exception handlers complete
==> default: [2014-07-23T13:49:24+00:00] FATAL: Stacktrace dumped to /var/chef/cache/chef-stacktrace.out
==> default: [2014-07-23T13:49:24+00:00] ERROR: rbenv_gem[2.1.2::berkshelf] (system) (rbenv::system line 46) had an error: Mixlib::ShellOut::CommandTimeout: Command timed out after 600s:
==> default: Command execeded allowed execution time, process terminated
==> default: ---- Begin output of export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" ----

...

==> default: ---- End output of export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" ----
==> default: Ran export RBENV_ROOT="/usr/local/rbenv" && export PATH="$RBENV_ROOT/bin:$RBENV_ROOT/shims:$PATH" && export RBENV_VERSION="2.1.2" && $RBENV_ROOT/shims/gem install berkshelf -q --no-rdoc --no-ri -v "3.1.4" returned 
==> default: [2014-07-23T13:49:24+00:00] FATAL: Chef::Exceptions::ChildConvergeError: Chef run process exited unsuccessfully (exit code 1)
```