Update composer.json

[sheetal1606]

Updated guzzle version in composer file

[minnur]

Would it be possible to make a release with Guzzle 7 support ?

composer require lullabot/amp:^2.0

Problem 1
- lullabot/amp[2.0.0, ..., 2.0.3] require guzzlehttp/guzzle ~6.1 -> found guzzlehttp/guzzle[6.1.0, ..., 6.5.8] but the package is fixed to 7.5.3 (lock file version) by a partial update and that version does not match. Make sure you list it as an argument for the update command.
- Root composer.json requires lullabot/amp ^2.0 -> satisfiable by lullabot/amp[2.0.0, 2.0.1, 2.0.2, 2.0.3].