Add integration with TravisCI

[webknjaz]

Please configure integration with TravisCI:

• [x] Enable repo testing in TravisCI
• [x] Add TravisCI badge (master branch) to README.md with a single commit. (click next to octocat icon on this page)
• [x] Create new branch called feature/travis-ci-integration and do all the following steps within that branch.
• [x] Add .travis.yml using pre-commit as a script. Set it up to run Python 3.5 env and add PIP caching. Ensure you install pre-commit before running tests:

  language: python
  python: 3.5
  cache: pip
  before_install: pip install -U pre-commit
  script: pre-commit run --all-files

• [x] Push the branch and create a Pull Request for it.