Start Amp only if PHP-Scoper is used

Starting Amp has an overhead and increase the likelyhood of rising an issue due to how Amp works. As
a result, it is better to not have to start it unless necessary. So far the current compactors are
fast enough to not require parallel processing, the build from Box without PHP-Scoper goes from 5s
with parallel processing to 3s without.

Closes box-project#135

src/Box.php

@@ -338,7 +338,7 @@ private function processContents(array $files, string $cwd): array
             return [$local, $processedContents];
         };
 
-        return is_parallel_processing_enabled()
+        return is_parallel_processing_enabled() && false === ($this->scoper instanceof NullScoper)
             ? wait(parallelMap($files, $processFile))
             : array_map($processFile, $files)
         ;