ruby_cmd for spawn
ruby_cmd for spawn

Use ruby_cmd instead of system ruby which may not exist.
Fix #288.

eregon/rubyspec,sgarciac/spec,ruby/rubyspec,ruby/spec,nobu/rubyspec,kachick/rubyspec,kachick/rubyspec,ruby/spec,sgarciac/spec,kachick/rubyspec,sgarciac/spec,eregon/rubyspec,nobu/rubyspec,eregon/rubyspec,ruby/rubyspec,nobu/rubyspec,ruby/spec