Update example to output a single line on PHP 7.2

Somewhere in the development leading up to PHP 7.2, var_dump()
started outputting the file and line number as part of it's output.
This causes one of the end-to-end tests to fail as the example script
it uses outputs twice as many lines as expected.

This fixes this by using var_export() and a little wrapping to output
useful debug data if something goes wrong and sensible output
otherwise.

examples/generating_integers.php

@@ -7,5 +7,5 @@
 $eris
     ->forAll(Generator\int())
     ->then(function ($integer) {
-        var_dump($integer);
+        echo var_export($integer, true) . PHP_EOL;
     });