remove osx from travis - dont work with php 7.0

.travis.yml

@@ -1,6 +1,5 @@
 os:
   - linux
-  - osx
 language: php
 php:
   - 7.0

src/Phpml/Classification/KNearestNeighbors.php

@@ -67,7 +67,7 @@ class KNearestNeighbors implements Classifier
      *
      * @throws \Phpml\Exception\InvalidArgumentException
      */
-    private function kNeighborsDistances(array $sample): array
+    private function kNeighborsDistances(array $sample)
     {
         $distances = [];