Introduces fixes to make test pass and suggest Travis CI

Hi there @rodjek,

I notice that a different fork of librarian-puppet is currently using [Travis CI (of their project)](https://travis-ci.org/maestrodev/librarian-puppet) to run the tests. But it was red.
I fixed the tests and now it is running ok.

The issue was that 'puppet' was made a soft dependency, but if you run prepending `bundle exec` to it, it wouldn't be found because it is not specified on the Gemfile.
Now it is showing up on the Gemfile as of the test group of dependencies.
I would like to see your opinion on this.

I would like to suggest together with this pull request to put librarian-puppet under [Travis CI](https://travis-ci.org)
This pull request contains a small change on the README including a picture of the latest build.