Fix failing specs on ree/ruby18
Fix failing specs on ree/ruby18

We put a conditional around tests so that we skip those those using
show-source (namely the one in run_command_spec.rb) on ree/ruby18

jasonm23/pry,raeno/pry,nju520/pry,sideci-sample/sideci-sample-pry,josue804/debunker,ooohiroyukiooo/pry,richo/pry,geoffreylitt/pry,sferik/pry,jasnow/pry,scalixte-mdsol/pry,shockone/pry,deivid-rodriguez/pry,jasnow/pry,raeno/pry,icebreaker/pry,jasonm23/pry,caporta/pry,caporta/pry,85x14/pry,pvdb/pry,icebreaker/pry,raeno/pry,bronzdoc/pry,nju520/pry,jasonm23/pry,scalixte-mdsol/pry,tdg5/pry,sferik/pry,printercu/pry,85x14/pry,jasnow/pry,tdg5/pry,printercu/pry,gfvcastro/pry,ngpestelos/pry,gfvcastro/pry,ooohiroyukiooo/pry,tdg5/pry,caporta/pry,85x14/pry,pvdb/pry,geoffreylitt/pry,josue804/debunker,scalixte-mdsol/pry,nju520/pry,shockone/pry,shockone/pry,ngpestelos/pry,ooohiroyukiooo/pry,bronzdoc/pry