Make stdout deprecation warning more specific

In a Rails application with a large number of dependencies, it's hard to
know which library is emitting a warning. This adds "capybara-webkit" to
the deprecation warning.

Resolves #607.

lib/capybara/webkit/connection.rb

@@ -16,7 +16,8 @@ module Capybara::Webkit
       if options.has_key?(:stderr)
         @output_target = options[:stderr]
       elsif options.has_key?(:stdout)
-        warn "[DEPRECATION] The `stdout` option is deprecated.  Please use `stderr` instead."
+        warn '[DEPRECATION] The Capybara::Webkit::Connection `stdout` option ' \
+          'is deprecated. Please use `stderr` instead.'
         @output_target = options[:stdout]
       else
         @output_target = $stderr