[FEATURE] Tests using travis

[rvock]

I've updated the composer.json and added a .travis.yml to enable testing using Travis CI.

To enable automatic tests, you need to activate the realurl-repository here: https://docs.travis-ci.com/user/getting-started/

Note: I've also changed typo3-cms-core requirement to "^6.2.14 || ^7.6 || ^8.0".

If you want the travis badge, you can add this to the README.md:

[![Build Status](https://travis-ci.org/dmitryd/typo3-realurl.svg?branch=master)](https://travis-ci.org/dmitryd/typo3-realurl)

(This image is for the branch on my fork)

The problem I find with using the badge is, that there is no support for automatic selection of the current branch. But you also could add two badges: One for master, one for development: http://stackoverflow.com/questions/32751929/get-travis-shield-on-github-to-reflect-selected-branch-status/32762503#32762503

[dmitryd]

While useful, it is not currently a priority. I have limited time, so I'd rather spend it on bugs and direct improvements. So priority for this is very low. Sorry about that.

[astehlik]

I started working on this before I found this pull request.

My version is running tests with the current TYPO3 master (taking the directory changes into account). Please feel free to use parts of my solution for inspiration :)

https://github.com/dmitryd/typo3-realurl/compare/development...Intera:feature/travis-ci?expand=1

A current build history can be found here:

https://travis-ci.org/Intera/typo3-extension-realurl/builds/202042285