Created unified system for running tests in travis for Piwik Core and any Piwik plugin.

System includes a console command (generate:travis-yml) that generates a .travis.yml file for a plugin or core. The travis file will check in travis if it needs to be updated, and if so it will fail the build and tell the user. If a specific command line option is specified supplying a github user token, the .travis.yml file will be updated within travis automatically.

Notes:

  - comments and non-travis sections will be preserved
  - includes logic for plugins to test themselves against latest stable version instead of just on master
  - the system is primarily controlled by environment variables. developers can change/add variables in .travis.yml and the auto-updating will preserve the changes.

This build is for a build w/ an out of date .travis.yml: https://travis-ci.org/piwik/piwik/builds/32146361
This build is the build that occurred right after the auto-update commit: https://travis-ci.org/piwik/piwik/builds/32146459

I will use the command on plugins after merged w/ core.
this function is also defined in https://github.com/piwik/piwik/blob/master/tests/angularjs/scripts/install-ubuntu.sh#L3-L22

This could be reused