Fix ruby warnings Config => RbConfig
Fix ruby warnings Config => RbConfig

Fixes warnings like:
  spec/unit/plugins/ruby_spec.rb:44:in `block in <top (required)>': Use
  RbConfig instead of obsolete and deprecated Config.

RbConfig is available at least as far back as 1.8.7.

Fodoj/ohai,jseely/ohai,numericillustration/ohai,phreakocious/ohai,375gnu/ohai,bougyman/ohai,higanworks/ohai,dreamnite/ohai,johnbellone/ohai,jbfavre/ohai,johnbellone/ohai,375gnu/ohai,davide125/ohai,bougyman/ohai,jaymzh/ohai,criteo-forks/ohai,criteo-forks/ohai,btm/ohai,http-418/ohai,numericillustration/ohai,tas50/ohai-1,shtzr840329/ohai,mattray/ohai,Fodoj/ohai,davide125/ohai,jseely/ohai,zshuo/ohai,chef/ohai,Juniper/ohai,MSOpenTech/ohai,higanworks/ohai,acaiafa/ohai,adamleff/ohai,glennmatthews/ohai,cmluciano/ohai,shtzr840329/ohai,http-418/ohai,dreamnite/ohai,tas50/ohai-1,n-marton/ohai,mattray/ohai,adamleff/ohai,jbfavre/ohai,juliandunn/ohai,Juniper/ohai,MSOpenTech/ohai,acaiafa/ohai,nathwill/ohai,zshuo/ohai,sh9189/ohai,jaymzh/ohai,glennmatthews/ohai,phreakocious/ohai,n-marton/ohai,btm/ohai,chef/ohai,nathwill/ohai,sh9189/ohai,juliandunn/ohai,cmluciano/ohai