Restore gunicorn in the list of third-party requirements
Restore gunicorn in the list of third-party requirements

`gunicorn==18.0` had previously been listed in requirements.txt, but I
removed it in commit d851a7e2a1a3239dd853ff88f918e1c52cf4b8f4 for
compatibility with the new httpbin.org hosting platform.

gunicorn 19.2 has (finally) been released, and this version will work
correctly for httpbin.org. This change adds `gunicorn==19.2` back to
requirements.txt.

This may also fix problems deploying to Heroku reported by @sigmavirus24,
which may have been caused by the requirement change in
d851a7e2a1a3239dd853ff88f918e1c52cf4b8f4.

SunGuo/httpbin,OndrejPontes/httpbin,mozillazg/bustard-httpbin,ashcoding/httpbin,luosam1123/httpbin,nkhuyu/httpbin,Stackato-Apps/httpbin,hnq90/httpbin,vscarpenter/httpbin,Runscope/httpbin,Stackato-Apps/httpbin,paranoiasystem/httpbin,bradparks/httpbin__http_echo_service_for_testing_http_requests,luhkevin/httpbin,scottydelta/httpbin,luosam1123/httpbin,postmanlabs/httpbin,yangruiyou85/httpbin,lioonline/httpbin,OndrejPontes/httpbin,pestanko/httpbin,logonmy/httpbin,phouse512/httpbin,mojaray2k/httpbin,mansilladev/httpbin,sigmavirus24/httpbin,gvangool/httpbin,pskrz/httpbin,bradparks/httpbin__http_echo_service_for_testing_http_requests,ashcoding/httpbin,fangdingjun/httpbin,Lukasa/httpbin,postmanlabs/httpbin,shaunstanislaus/httpbin,yangruiyou85/httpbin,marinehero/httpbin,gvangool/httpbin,fangdingjun/httpbin,pestanko/httpbin,krissman/httpbin,admin-zhx/httpbin,kennethreitz/httpbin,hnq90/httpbin,sigmavirus24/httpbin,yemingm/httpbin,yemingm/httpbin,Jaccorot/httpbin,SunGuo/httpbin,habibmasuro/httpbin,admin-zhx/httpbin,Runscope/httpbin,ewdurbin/httpbin,kennethreitz/httpbin,scottydelta/httpbin,habibmasuro/httpbin,mozillazg/bustard-httpbin,Jaccorot/httpbin,Lukasa/httpbin,krissman/httpbin,pskrz/httpbin,vscarpenter/httpbin,luhkevin/httpbin,nkhuyu/httpbin,lioonline/httpbin,ewdurbin/httpbin,marinehero/httpbin,logonmy/httpbin,paranoiasystem/httpbin,mansilladev/httpbin,shaunstanislaus/httpbin,phouse512/httpbin,mojaray2k/httpbin