Part 1 of https://github.com/piwik/piwik/issues/8373
Moves the travis scripts and files and the .travis.yml generator to a new repository so it is decoupled from Piwik. This way, changes in Piwik won't cause the travis files or generator to fail, and for older Piwiks the travis scripts can be patched to make the build work by creating version branches.
The way the system works now is as follows: - .travis.yml is generated using the code ./tests/travis/generator which does not depend on Piwik. This generated file must always work with supported Piwik versions, no matter which version generated it. - During travis execution, the autoupdate check for .travis.yml will be done before any other operation using the latest travis-scripts code. This means the latest version of .travis.yml will be used, which should work since we require it to work w/ all supported Piwik versions. - In plugin builds when checking out the Piwik branch to test against, we will afterwards force the master branch in travis-scripts to be used. If there is a branch in travis-scripts that matches the version of Piwik the build is testing against, however, that branch is used. - After this the build proceeds as normal.