Default `output_stream` to $stdout.

This is what the docs say we do. It's also necessary
so that if `configuration.reporter` is accessed before
the CommandLine class has set the output_stream, it
uses $stdout as its output rather than a `StringIO.new`.

Before, issuing a deprecation from `option_parser.rb`
was causing no spec output at all since that happened
before the output_stream was set.
Fix a couple specs so that they explicitly redirect unwanted output.

These specs depended on the reporter defaulting
to a `StringIO.new`. Now that it is defaulting to
`$stdout`, we were getting unwanted output from these
two specs.
Use `output_stream` as the standardized method name.

Everything else in RSpec uses `output_stream`.
Warn if output_stream is changed after it has already been used by the reporter.
Remove deprecated `out` and `output` methods.
Add changelog entry.
Revert "Swap output stream to current sandbox (Read why)."

This reverts commit a22d52467d936ff7bac9860b9d8275f9d0b55e43.

This is no longer needed with the new approach we are taking
to `output_stream` (e.g. having it default to $stdout), and
it was causing `command_line_spec.rb` to have a failure when
run in isolation.
Stub `warn` to silence "output stream already used" warning.

We don't care about it here.
Merge pull request #1142 from rspec/fix-1127-in-master

Fix 1127 in master