[MINOR][ML] fix compile warns

This fixes some compile time warnings.

Author: Xiangrui Meng <meng@databricks.com>

Closes #9319 from mengxr/mllib-compile-warn-20151027.

mllib/src/main/scala/org/apache/spark/ml/regression/LinearRegression.scala

@@ -145,7 +145,7 @@ class LinearRegression(override val uid: String)
     // Extract the number of features before deciding optimization solver.
     val numFeatures = dataset.select(col($(featuresCol))).limit(1).map {
       case Row(features: Vector) => features.size
-    }.toArray()(0)
+    }.first()
     val w = if ($(weightCol).isEmpty) lit(1.0) else col($(weightCol))
 
     if (($(solver) == "auto" && $(elasticNetParam) == 0.0 && numFeatures <= 4096) ||

mllib/src/test/scala/org/apache/spark/ml/classification/LogisticRegressionSuite.scala

@@ -17,6 +17,7 @@
 
 package org.apache.spark.ml.classification
 
+import scala.language.existentials
 import scala.util.Random
 
 import org.apache.spark.SparkFunSuite
@@ -24,7 +25,7 @@ import org.apache.spark.ml.feature.Instance
 import org.apache.spark.ml.param.ParamsSuite
 import org.apache.spark.ml.util.MLTestingUtils
 import org.apache.spark.mllib.classification.LogisticRegressionSuite._
-import org.apache.spark.mllib.linalg.{Vectors, Vector}
+import org.apache.spark.mllib.linalg.{Vector, Vectors}
 import org.apache.spark.mllib.regression.LabeledPoint
 import org.apache.spark.mllib.util.MLlibTestSparkContext
 import org.apache.spark.mllib.util.TestingUtils._