Update PHP buildpack

[jmcarp]

Based on https://www.us-cert.gov/ncas/bulletins/SB16-263, we need to update to php buildpack > 4.3.18. We can either:

• Upgrade to cf-release 242
• Manually update php buildpack
• [x] Legacy CF Production
• [ ] Legacy CF Staging
• [x] GovCloud CF Production
• [x] GovCloud CF Staging

[sharms]

` curl -L -O https://github.com/cloudfoundry/php-buildpack/releases/download/v4.3.20/php_buildpack-cached-v4.3.20.zip

cf update-buildpack php_buildpack -p ./php_buildpack-cached-v4.3.20.zip`

[sharms]

Legacy staging is currently completely unavailable and as such is not exposed. Rolling out 242 will resolve.