Travis CI finalization

[deniztoprak]

Travis continuous integration is introduced with #20 and it runs expectedly. However, the build process is failing because of missing Docker Hub / GitHub credentials. An authorized user should follow these 2 steps:

1. To deploy project images on Docker Hub registry, an authorized Docker user must set his/her login credentials in 2 environment variables, as mentioned on Travis documentation. DOCKER_USERNAME and DOCKER_PASSWORD have to be defined on the settings page. Display value in build log option must be switched off and the Docker user must have a write access on the official Docker Hub repository.

2. Semantic release module requires also push access on GitHub repository in order to create tags and releases. An authorized GitHub user has to generate a personal access token which has to be saved in GH_TOKEN variable on Travis settings.

.travis.yml file doesn't need to be changed.

Travis settings should look like this:

Build and deployment process take up around 20 minutes. They could be improved with Docker Layer caching, but Travis discourages it. I still tried CACHE_FROM directive but it didn't work as expected. The images are built every single time from scratch.

[marsmensch]

@froid1911 please update the Docker credentials in the travis-ci settings (and make sure to remove my changes in c1aac23 before ;-) Thank you!