spec/ruby/core/unboundmethod/bind_call_spec.rb: Add ruby_version_is guard

spec/ruby/core/unboundmethod/bind_call_spec.rb

@@ -1,7 +1,8 @@
 require_relative '../../spec_helper'
 require_relative 'fixtures/classes'
 
-describe "UnboundMethod#bind_call" do
+ruby_version_is '2.7' do
+  describe "UnboundMethod#bind_call" do
     before :each do
       @normal_um = UnboundMethodSpecs::Methods.new.method(:foo).unbind
       @parent_um = UnboundMethodSpecs::Parent.new.method(:foo).unbind
@@ -47,4 +48,5 @@ describe "UnboundMethod#bind_call" do
       um = p.method(:singleton_method).unbind
       ->{ um.bind_call(other) }.should raise_error(TypeError)
     end
+  end
 end