TravisCI: Test with different HttpClient versions
TravisCI: Test with different HttpClient versions

fdagostini/HTTP-Proxy-Servlet,cr4igo/FirstSpirit-HTTP-Proxy-FSM,mitre/HTTP-Proxy-Servlet,jpenninkhof/HTTP-Proxy-Servlet,cthiebaud/HTTP-Proxy-Servlet,rmohta/HTTP-Proxy-Servlet,hrbapna/HTTP-Proxy-Servlet,wchen057/HTTP-Proxy-Servlet,cthiebaud/HTTP-Proxy-Servlet,jziub/HTTP-Proxy-Servlet,1990balaji/HTTP-Proxy-Servlet