Update rack, nokogiri, stop using platform-api fork

[jalada]

Update rack and nokogiri due to a bunch of CVEs:

• https://nvd.nist.gov/vuln/detail/CVE-2018-16471
• https://nvd.nist.gov/vuln/detail/CVE-2016-4658
• https://nvd.nist.gov/vuln/detail/CVE-2017-5029
• https://nvd.nist.gov/vuln/detail/CVE-2018-14404
• https://nvd.nist.gov/vuln/detail/CVE-2017-18258
• https://nvd.nist.gov/vuln/detail/CVE-2017-9050

Additionally, the Heroku platform-api gem now supports API calls for modifying SNI endpoint certificates, so our fork is no longer required.