move dependency check from runtime to mrbgem.rake

mrbgems/mruby-enumerator/mrbgem.rake

 MRuby::Gem::Specification.new('mruby-enumerator') do |spec|
   spec.license = 'MIT'
   spec.author  = 'mruby developers'
+  spec.add_dependency('mruby-fiber')
 end

mrbgems/mruby-enumerator/mrblib/enumerator.rb

@@ -346,8 +346,6 @@ class Enumerator
     end
     raise @stop_exc if @stop_exc
 
-    raise NotImplementedError, "Enumerator require Fiber" unless Object.const_defined?(:Fiber)
-
     curr = Fiber.current
 
     if !@fib || !@fib.alive?