only clean resolution-cache if it exists
only clean resolution-cache if it exists

yannickdeturck/heroku-buildpack-scala,Javilete/buildpack,shunjikonishi/heroku-buildpack-scala-compiled,Scalingo/scala-buildpack,yannickdeturck/heroku-buildpack-scala,agideon/heroku-buildpack-scala,KimStebel/heroku-buildpack-scala,shunjikonishi/heroku-buildpack-scala-compiled,Scalingo/scala-buildpack,heroku/heroku-buildpack-scala,phdezann/heroku-buildpack-scala,hpcloud/heroku-buildpack-scala,Javilete/buildpack,agideon/heroku-buildpack-scala,phdezann/heroku-buildpack-scala,hpcloud/heroku-buildpack-scala,heroku/heroku-buildpack-scala,KimStebel/heroku-buildpack-scala