Added a file check for plugins.sbt in play detection
Added a file check for plugins.sbt in play detection

heroku/heroku-buildpack-scala,heroku/heroku-buildpack-scala,agideon/heroku-buildpack-scala,Scalingo/scala-buildpack,agideon/heroku-buildpack-scala,Scalingo/scala-buildpack,yannickdeturck/heroku-buildpack-scala,yannickdeturck/heroku-buildpack-scala