Can't find ruby 2.1.0's source dir

It seems the logic for locating the source directory strongly expects that there is a `-pNN` version suffix. This piece of code (extracted & polished up from the `debugger` gem source) should, I believe, create a (meaningless) Makefile, but instead fails:

```ruby
require 'mkmf'
require 'debugger/ruby_core_source'
Debugger::RubyCoreSource.create_makefile_with_core(lambda{}, "foo")
```

Outputs:
```
Makefile creation failed
**************************************************************************
No source for ruby-2.1.0-p0 provided with debugger-ruby_core_source gem.
**************************************************************************
```