enable travis to run tests on PHP 5.6 to see if a reported problem only occurs on 5.6. It will be released soon

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.

example failed tests https://travis-ci.org/piwik/piwik/jobs/32095780

Fixes #5807, refactor CronArchive class so it does not configure itself based on command line arguments. It now has properties that must be set. The core:archive command has been refactored to set these properties based on command line args.

Also includes:

  - bug fix to CronArchive::initLog method (config was modified after log instance created which negated it's effect)
  - misc/archive.php no longer uses Archive.php but executes the core:archive console command using Piwik\Console
  - remove forcelogtoscreen parameter (no longer necessary due to initLog changes)

eg. https://travis-ci.org/piwik/piwik/jobs/29180844

https://travis-ci.org/piwik/piwik/jobs/28488319

eg. https://travis-ci.org/piwik/piwik/jobs/28309237