Introduced Composer vendors

Removed the following libraries from the code and added the dependencies to the composer.json file:
* PHPExcel
* TCPDF
* Geoip
* AdoDB

There are some libraries that can't be moved right now.

I've tested it and it works OK. Some libraries had to be updated as they were too old to be available in Packagist...

Tasks done in this PR:
* Updated .travis.yml file to install vendors
* Created a new constant DOLIBARR_VENDOR_ROOT that maps to the vendor folder
* Updated filefunc.inc.php to use DOLIBARR_VENDOR_ROOT instead of mapping to /htdocs/includes for the libraries above mentioned.