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.

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