To read from relative path on the system use System.getProperty method to get current directory and further uses to load the file using relative path.
scala> val path = System.getProperty("user.dir").concat("/../2015-summary.csv")
scala> val csvDf = spark.read.option("inferSchema","true").option("header", "true").csv(path)
scala> csvDf.take(3)
spark:2.4.4 scala:2.11.12