Upgrade compromised dependencies

[macoca]

Several security vulnerabilities have been reported in the last months.

• Upgrade rack to version 2.0.6 or later.
  gem "rack", ">= 2.0.6"
  https://nvd.nist.gov/vuln/detail/CVE-2018-16471

• Upgrade loofah to version 2.2.3 or later.
  gem "loofah", ">= 2.2.3"
  https://nvd.nist.gov/vuln/detail/CVE-2018-16468

• Upgrade rubyzip to version 1.2.2 or later.
  gem "rubyzip", ">= 1.2.2"https://nvd.nist.gov/vuln/detail/CVE-2018-1000544

• Upgrade ffi to version 1.9.24 or later.
  gem "ffi", ">= 1.9.24"
  https://nvd.nist.gov/vuln/detail/CVE-2018-1000201