Adds Kernel#require_relative

I'm new to working with the rubyspecs, but here are my results from running the following command on these branches:
`rake spec:run[spec/rubyspec/core/kernel/require_relative_spec.rb]`

`master`: `1 file, 42 examples, 22 expectations, 20 failures, 30 errors`
`johnnyt/require_relative`: `1 file, 42 examples, 96 expectations, 0 failures, 0 errors`

Do I need to do anything with `rake spec:untag` / `rake spec:tag[file]`?