Wait to configure expose_dsl_globally until RSpec is fully defined.

Some config methods use `RSpec.world` and have problems if `RSpec.world`
has not been defined yet. So it's best to wait to configure anything
until `RSpec` is defined.

Author: myronmarston

lib/rspec/core.rb

@@ -83,7 +83,6 @@ def self.clear_examples
   def self.configuration
     @configuration ||= RSpec::Core::Configuration.new
   end
-  configuration.expose_dsl_globally = true
 
   # Yields the global configuration to a block.
   # @yield [Configuration] global configuration
@@ -178,4 +177,7 @@ def self.const_missing(name)
     require MODULES_TO_AUTOLOAD.fetch(name) { return super }
     ::RSpec.const_get(name)
   end
+
+  Core::DSL.expose_globally!
+  Core::SharedExampleGroup::TopLevelDSL.expose_globally!
 end