add support for compileString() method in scssphp 1.5 and above

The SCSS task uses Compiler::compile() which is deprecated in current
scssphp. Add support for new compileString() method which returns a
result object instead of string.

src/Task/Assets/Scss.php

@@ -64,7 +64,12 @@ protected function scssphp($file)
             $scss->setFormatter($this->compilerOptions['formatter']);
         }
 
-        return $scss->compile($scssCode);
+        if (method_exists($scss, 'compileString')) {
+            // "compileString()" is available since scssphp v1.5
+            return $scss->compileString($scssCode)->getCss();
+        } else {
+            return $scss->compile($scssCode);
+        }
     }
 
     /**