Simplify test infrastructure

* Manage dependencies with Cask
* Add a Vagrantfile and a shell script for provisioning
* Simplify Makefile
* Add a script for running the tests in batch mode

The one thing that remains to be done is to reintegrate the batch
checkdoc check.
Replace problematic cl-position with custom code

`cl-position` in Emacs 24.3 is not the same as the one in the cl-lib
library on GNU ELPA, which is causing problems for people using older
Emacsen. This commit replaces it with a bit of custom code.

This fixes #356.
Add a note on running the tests
Remove elpa file in clean task
Fix broken Travis build