Use ruby-debug by default in --spec-debug.
Use ruby-debug by default in --spec-debug.

The default --spec-debug is overridden in Rubinius spec/custom to
use the reference debugger. Since the defaults in mspec and rubyspec
are intended to work with MRI and ruby-debug is basically the standard
on MRI, we use that as the default.

kachick/mspec,rdp/mspec,elia/mspec,timfel/mspec,voxik/mspec,iainbeeston/mspec,shirosaki/mspec,opal/mspec,alex/mspec,shugo/mspec,iainbeeston/mspec,ruby/mspec,kachick/mspec,MagLev/mspec,ryoqun/mspec,rubinius/mspec,opal/mspec,ruby/mspec,MagLev/mspec,Gibheer/mspec,Zoxc/mspec,mkb/mspec,ryoqun/mspec,eregon/mspec,nobu/mspec