fix errors on hhvm with float casting

src/Phpml/FeatureExtraction/TfIdfTransformer.php

@@ -32,7 +32,7 @@ class TfIdfTransformer implements Transformer
 
         $count = count($samples);
         foreach ($this->idf as &$value) {
-            $value = log($count / $value, 10);
+            $value = log(floatval($count / $value), 10.0);
         }
     }
 

src/Phpml/Preprocessing/Normalizer.php

@@ -78,7 +78,7 @@ class Normalizer implements Preprocessor
         foreach ($sample as $feature) {
             $norm2 += $feature * $feature;
         }
-        $norm2 = sqrt($norm2);
+        $norm2 = sqrt(floatval($norm2));
 
         if (0 == $norm2) {
             $sample = array_fill(0, count($sample), 1);