Added build status icon to README.md
Added build status icon to README.md

Followed guide from here: http://about.travis-ci.org/docs/user/status-images/

Current icon shows passing build
innokenty/zt-zip,zeroturnaround/zt-zip,liyi828328/zt-zip,hgl888/zt-zip,lxhxhlw/zt-zip,zeroturnaround/zt-zip,innokenty/zt-zip,lxhxhlw/zt-zip,myroid/zt-zip,myroid/zt-zip,hgl888/zt-zip,liyi828328/zt-zip