Yasnippet: avoid yasmate submodule, and remove hacks

* Fixed #1511 by using `git submodule` in `:build` instead of
  `:submodules`.
* Removed the complicated looking customize hacks that aren't needed
  anymore, as far as I can tell.
* According to #200 there is some problem with compilation, although I
  couldn't see exactly what the error was from there. I added
  `yasnippet.el` to `:compile` and eveything appears to be working
  fine when tested.