Add travis yml

[cormier]

Following the discussion in #33, this PR creates one Dockerfile per supported OJS version and adds CI to the repo (this involved reverting the changes made in #32 -- sorry for the confusion!). The CI tests that the docker images can be built correctly, but do not perform additional tests. The PR also add a build status badge to the README.

I don't know exactly which versions we want to support, so I have added the last two versions. The Dockerfiles were created manually assuming that nothing in them changes except the version of OJS. If we decide to add flavours (e.g. php5, php7), we can add an update.sh script to generate the Dockerfiles. In the meantime, I believe we can manage that manually.

Here is some work I would like to do to bring this furtherr:

• [ ] Add a generate-stackbrew-library.sh script to generate a list of tags for the repo, following the best practices described here
• [ ] Activate automatic image building on the docker hub repo

[cormier]

@lucasdiedrich if you accept to merge this PR, you would have to activate travis on this repo, here

[marcbria]

This looks great @cormier!! Great work!!