Fix handling of embed for the JSON formatter

The legacy_formatter need to make sure that the embed calls from the
step definitions are not passed to the formatter until it got
information about the corresponding step.
Add @spawn to the scenario: embedding screenshot

If the scenario is not @spawn:ed the scenario: "
features/docs/gherkin/background.feature:346 #
Scenario: run a feature with a background that is pending"
fails in the JRuby build on Travis (due to that a full error backtrace
is emitted, so it seems that the -b flag is carried over somehow).
Merge branch 'json-formatter-embed' of github.com:brasmusson/cucumber into brasmusson-json-formatter-embed

Conflicts:
	lib/cucumber/reports/legacy_formatter.rb