TYPO3 Core

According to the "Pre-Merge-Tests" for TYPO3 (Gerrit -> Travis CI and back) it make sense to add the Travis CI Build badge to the README.

It is not a big feature, but a nice small addition for the TYPO3-Syncs from git.typo3.org to github like

• https://github.com/TYPO3/TYPO3.CMS
• https://github.com/typo3-ci/TYPO3.CMS-pre-merge-tests

Further more this a small task at the road to get Pre-Merge-Tests ready.
See https://docs.google.com/document/d/1oTSRUf1nfBHK2FbPwV3D3kOxzKY7zjEzSHrVqpWVFe4/edit#heading=h.jiz2bmhuh8gb

This can be easily included in master, the next patch level release and back ported into every supported version.