Add Travis CI config file so that builds work properly

I noticed that Travis CI was enabled for this repo but the [builds are failing because a ```.travis.yml``` file is not present to configure it](https://travis-ci.org/qos-ch/logback-extensions), causing the builds to default to trying to use ruby.

This pull request:
- Adds the missing ```.travis.yml``` config file so that all pushes to this repo will be properly built on travis.
 - It currently uses openjdk7 and oraclejdk7 (the project does not support 1.6 according to the pom)
-- See my Travis CI builds for this forked branch here: https://travis-ci.org/ericdahl/logback-extensions/builds/20326756
- Updates the README to be markdown syntax and adds the Build Status badge to it
 - See example on my forked repo here: https://github.com/ericdahl/logback-extensions/tree/travis-ci

Also... logback itself seems to be configured to use [CloudBees](https://logback.ci.cloudbees.com/job/logback/) and is not configured to be using [Travis CI](https://travis-ci.org/qos-ch/logback) so I'm a bit uncertain about which is preferred.