Add missing 'cgi' require in push client

[Sinjo]

Fixes #202

Tested the fix on 2.7.2 with the push gateway running locally and it seems to do the trick:

sinjo@coin:~/projects/client_ruby$ bx irb
irb(main):001:0> require "prometheus/client/push"
=> true
irb(main):002:0> registry = Prometheus::Client.registry
=> #<Prometheus::Client::Registry:0x00005620d98717e8 @metrics={}, @mutex=#<Thread::Mutex:0x00005620d9871658>>
irb(main):003:0> Prometheus::Client::Push.new('test-job').add(registry)
=> #<Net::HTTPOK 200 OK readbody=true>

[coveralls]

Coverage remained the same at 100.0% when pulling 4f95dd0 on sinjo-add-missing-require into 76e3cb8 on master.

[coveralls]

Coverage remained the same at 100.0% when pulling 4f95dd0 on sinjo-add-missing-require into 76e3cb8 on master.