docs: add Travis CI badge to readme
docs: add Travis CI badge to readme

Makes it easy to check current build status.

erdi/CodeNarc,darxriggs/CodeNarc,erdi/CodeNarc,CodeNarc/CodeNarc,CodeNarc/CodeNarc,darxriggs/CodeNarc,erdi/CodeNarc,erdi/CodeNarc