Add TravisCI build status on readme

[albertotn]

Hi! Thanks for your project! I've seen you have put jongo also on TravisCI and because there is a lot of "green status" on build history (see https://travis-ci.org/bguerout/jongo/builds ), why you don't add on readme I mean this small image: https://api.travis-ci.org/bguerout/jongo.svg?branch=master

Keep going with this good project!

[bguerout]

Thanks