update composer dependencies
update composer dependencies

Changelogs summary:

 - guzzlehttp/guzzle updated from 6.3.0 to 6.3.2
   See changes: https://github.com/guzzle/guzzle/compare/6.3.0...6.3.2
   Release notes: https://github.com/guzzle/guzzle/releases/tag/6.3.2

 - phpunit/phpunit updated from 7.0.2 to 7.0.3
   See changes:
https://github.com/sebastianbergmann/phpunit/compare/7.0.2...7.0.3
   Release notes:
https://github.com/sebastianbergmann/phpunit/releases/tag/7.0.3

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