Remove dependency on cgi

This was introduced because `cgi` was moved out of the default gems in
4.0. It turns out we can simply use `uri`, which still ships with Ruby,
for our purposes instead.

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

Author: Sinjo

prometheus-client.gemspec

@@ -16,7 +16,6 @@ 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'