Look for Quicklisp in ~/quicklisp and ~/.quicklisp

In `prelude-common-lisp.el`, look in both `~/quicklisp/` and
`~/.quicklisp/` for `slime-helper.el`, on the assumption that most users
will have Quicklisp installed in one of these two locations.

This commit fixes #582
Merge pull request #587 from swhalen/two-quicklisp-dirs

Look for Quicklisp in ~/quicklisp and ~/.quicklisp