Add cgi as an explicit dependency

It's no longer a default gem in 4.0, so we need to add it explicitly.

Signed-off-by: Chris Sinjakli <chris@sinjakli.co.uk>

Author: Sinjo

prometheus-client.gemspec

@@ -16,6 +16,7 @@ Gem::Specification.new do |s|
   s.require_paths     = ['lib']
 
   s.add_dependency 'base64'
+  s.add_dependency 'cgi'
 
   s.add_development_dependency 'benchmark'
   s.add_development_dependency 'benchmark-ips'