JBEHAVE-878 fixed build status image to point to official repo.

(Unfortunately there's no way to merge this pull request with the previous one  -- https://github.com/jbehave/jbehave-core/pull/35).

I've fixed the build status image in README, it is now pointing to jbehave/jbehave-core.

To enable Travis builds for jbehave/jbehave-core you'll need to follow some steps on your side too:
http://about.travis-ci.org/docs/user/getting-started/#Step-one%3A-Sign-in

as you can see there, the .travis.yml file is required to be in the repo's root directory.

Thanks