Update previous Spark version in Mima.

Author: rxin

project/MimaBuild.scala

@@ -91,7 +91,7 @@ object MimaBuild {
 
   def mimaSettings(sparkHome: File, projectRef: ProjectRef) = {
     val organization = "org.apache.spark"
-    val previousSparkVersion = "1.4.0"
+    val previousSparkVersion = "1.5.0"
     val fullId = "spark-" + projectRef.project + "_2.10"
     mimaDefaultSettings ++
     Seq(previousArtifact := Some(organization % fullId % previousSparkVersion),