Fix zip() for PHP 5.5

finally just doesn't work as expected in 5.5.

src/Observable/functions.php

@@ -200,10 +200,15 @@ function ($value) use (&$i, &$next, &$delayed, $key, $count, $emit) {
             try {
                 yield Awaitable\choose($coroutines);
                 yield $delayed;
-            } finally {
+
                 foreach ($coroutines as $coroutine) {
                     $coroutine->cancel();
                 }
+            } catch (\Exception $exception) {
+                foreach ($coroutines as $coroutine) {
+                    $coroutine->cancel($exception);
+                }
+                throw $exception;
             }
 
             yield $i; // Return the number of times a set was emitted.