Remove PHP5.x from CI system for PRs for NextCloud 14

[vwbusguy]

NextCloud 14.0.0 only supports PHP 7.0, 7.1, and 7.2, but the CI system for Pull Requests is still attempting to test NextCloud against PHP 5.x. This is an invalid test case as NC17 no longer supports <PHP 7.0.

References:

• https://nextcloud.com/blog/nextcloud-14-now-available-with-video-verification-signaltelegram-2fa-support-improved-collaboration-and-gdpr-compliance/
• https://nextcloud.com/blog/changes-for-developers-in-nextcloud-14-beta-4-ready-for-testing/
• PHP 7.0+ for Nextcloud 14 server#7368

[vwbusguy]

Example of false positive failure as a result of this: https://travis-ci.org/nextcloud/passman/jobs/390034499

[vwbusguy]

It looks like this has since been fixed. This may be closed.