Add missing method to RSpec3 formatter

[temyers]

Hi, this is a bit of a pre-emptive pull request, as I'd like a bit of guidance, and wasn't sure what to put into an issue, or what the specs should be.

I encountered the following issue when adding the ci_reporter to a project that contains specs that executes rake and specifically looks for the --format documentation output

 +/home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/formatters.rb:186:in `block in duplicate_formatter_exists?': undefined method `output' for #<CI::Reporter::RSpec3::Formatter:0x0000000266db98> (NoMethodError)
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/formatters.rb:185:in `any?'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/formatters.rb:185:in `duplicate_formatter_exists?'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/formatters.rb:178:in `register'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/formatters.rb:147:in `add'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/configuration.rb:828:in `add_formatter'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:117:in `block in load_formatters_into'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:117:in `each'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:117:in `load_formatters_into'
       +	from /home/vagrant/.rvm/gems/ruby-2.2.3/gems/rspec-core-3.5.4/lib/rspec/core/configuration_options.rb:23:in `configure'

I have a spec file containing:

RSpec.describe "My project" do
  def command(cmd)
    Bundler.with_clean_env do
      # Redirect stderr to stdout for simple command execution
      return `#{cmd} 2>&1`
    end
  end

  # ...

  it "should output with --format documentation" do
        expect(command("rake spec")).to include "should package myApp (FAILED - 1)"
   end

end

executing rake or rspec directly works as expected, but with the ci_reporter it fails.

This is only a partial implementation, since with the patch, the tests still fail because the documentation formatter is not used.

Should this be supported?