Change installation directory for templates

This change combined with a change in phpDocumentor/phpDocumentor2 (pull request on its way) should fix https://github.com/phpDocumentor/phpDocumentor2/issues/597 for good.

When phpDocumentor is vendored the templates will now be installed to something like ```vendor/phpdocumentor/templates```
There is no change to the installation location when installing dependencies for phpDocumentor itself.