change array_push() to $array[] (#254)

http://php.net/manual/en/function.array-push.php#refsect1-function.array-push-description

> Note: If you use array_push() to add one element to the array it's better to use $array[] = because in that way there is no overhead of calling a function.

Author: mosaxiv

docs/topics/recipes.md

@@ -856,8 +856,8 @@ $conditional2->getStyle()->getFont()->getColor()->setARGB(\PhpOffice\PhpSpreadsh
 $conditional2->getStyle()->getFont()->setBold(true);
 
 $conditionalStyles = $spreadsheet->getActiveSheet()->getStyle('B2')->getConditionalStyles();
-array_push($conditionalStyles, $conditional1);
-array_push($conditionalStyles, $conditional2);
+$conditionalStyles[] = $conditional1;
+$conditionalStyles[] = $conditional2;
 
 $spreadsheet->getActiveSheet()->getStyle('B2')->setConditionalStyles($conditionalStyles);
 ```

samples/Basic/08_Conditional_formatting.php

@@ -79,9 +79,9 @@
 $conditional3->getStyle()->getNumberFormat()->setFormatCode(NumberFormat::FORMAT_CURRENCY_EUR_SIMPLE);
 
 $conditionalStyles = $spreadsheet->getActiveSheet()->getStyle('B2')->getConditionalStyles();
-array_push($conditionalStyles, $conditional1);
-array_push($conditionalStyles, $conditional2);
-array_push($conditionalStyles, $conditional3);
+$conditionalStyles[] = $conditional1;
+$conditionalStyles[] = $conditional2;
+$conditionalStyles[] = $conditional3;
 $spreadsheet->getActiveSheet()->getStyle('B2')->setConditionalStyles($conditionalStyles);
 
 //	duplicate the conditional styles across a range of cells

samples/Basic/08_Conditional_formatting_2.php

@@ -55,8 +55,8 @@
 $conditional3->getStyle()->getFont()->getColor()->setARGB(Color::COLOR_GREEN);
 
 $conditionalStyles = $spreadsheet->getActiveSheet()->getStyle('A1')->getConditionalStyles();
-array_push($conditionalStyles, $conditional1);
-array_push($conditionalStyles, $conditional3);
+$conditionalStyles[] = $conditional1;
+$conditionalStyles[] = $conditional3;
 $spreadsheet->getActiveSheet()->getStyle('A1')->setConditionalStyles($conditionalStyles);
 
 //	duplicate the conditional styles across a range of cells

tests/PhpSpreadsheetTests/Writer/Xls/WorkbookTest.php

@@ -86,7 +86,7 @@ public function providerAddColor()
 
         // Add new color after all existing color
         $colorsAddTwo = array_map([$this, 'paletteToColor'], $palette);
-        array_push($colorsAddTwo, $this->paletteToColor($newColor1));
+        $colorsAddTwo[] = $this->paletteToColor($newColor1);
         $paletteTestSix = $palette;
 
         // Add one existing color