add microsecond in timestamp representation, for PHP 7.1

[remicollet]

Discovered in Fedora QA, since PHP 7.1.0RC6
See: https://apps.fedoraproject.org/koschei/package/php-akamai-open-edgegrid-auth?collection=f26

PHPUnit 5.6.2 by Sebastian Bergmann and contributors.
Runtime:       PHP 7.1.0RC6
Configuration: /builddir/build/BUILD/AkamaiOPEN-edgegrid-php-1617cf4bdeba7b5c46a1d55bb969d0e45c7f52f5/phpunit.xml
..F...................................                            38 / 38 (100%)
Time: 2.04 seconds, Memory: 4.00MB
There was 1 failure:
1) Akamai\Open\EdgeGrid\Tests\Client\Authentication\TimestampTest::testTimestampFormat
Failed asserting that two DateTime objects are equal.

This simple fix add microsecond in timestamp converted to string.
But I don't know if some usage to this library will be broken by this change.
So please review.

/cc @siwinski