It happens pretty often that travis fails with:
Could not fetch https://api.github.com/repos/sebastianbergmann/comparator/zipball/2b7424b55f5047b47ac6e5ccb20b2aea4011d9be, please create a GitHub OAuth token to go over the API rate limit
Head to https://github.com/settings/tokens/new?scopes=repo&description=Composer+on+travis-job-eb7eb7dc-8777-4fcb-bc85-43e142e5599a+2020-03-24+1113 to retrieve a token. It will be stored in "/root/.composer/auth.json" for future use by Composer.
https://github.com/composer/composer/issues/1314 has some insights what may help.