Update composer.json constraints for amp-library

Currently the drupal amp module requires amp-library version 1.0.* but its better if the constraint is a minimum constraint rather than a match constraint i.e. ^1.0.0-RC1. (This means >= 1.0.0-RC1 but <= 2.0.0)

The Drupal amp module has reached RC1 so its also fitting that it requires an RC1 library (at minimum).