Skip specs with non-mri-compatible backtrace. Also see rspec-support

The code in spec/support/ruby_features.rb shoul be moved into rspec-support

lib/rspec/support/spec/prevent_load_time_warnings.rb:53
-  it "issues no warnings when the spec files are loaded", :slow do
+  it "issues no warnings when the spec files are loaded", :slow, :unless => (RUBY_PLATFORM == 'java' || (defined?(RUBY_ENGINE) && RUBY_ENGINE == 'rbx')) do
     expect_no_warnings_from_files_in "spec", "require 'rspec/core'; require 'spec_helper'"
   end