Always return absolute path from `projectile-project-root`

Fixes #305 

Not sure if its the appropriate fix but it works.

By the way, can you explain what `acc` in the following snippet is??? It confuses the hell out of me

```scheme
(defun projectile-project-root ()
  "Retrieves the root directory of a project if available.
The current directory is assumed to be the project's root otherwise."
  (file-truename
   (let ((dir (file-truename default-directory)))
     (or (--reduce-from
          (or acc (funcall it dir)) nil
          projectile-project-root-files-functions)
         (if projectile-require-project-root
             (error "You're not in a project")
           default-directory)))))
```