Use with-eval-after-load instead of eval-after-load.

Introduced in 24.4, this is a better option to use as it is a macro. There seems to be weird behavior for `eval-after-load` when `swift-mode.el` is byte-compiled, which is fixed by us using `with-eval-after-load`. FWIW: the `BODY` in `eval-after-load` is not byte compiled vs. `with-eval-after-load`:

  - http://stackoverflow.com/questions/21880139/what-is-with-eval-after-load-in-emacs-lisp
  - http://debbugs.gnu.org/cgi/bugreport.cgi?bug=16810

*NOTE: This does not work on Emacs older than 24.4*. I honestly don't know how to make it backwards-compatible (outside of duplicating the `with-eval-after-load` macro inside of `swift-mode.el`; comments welcome.