Replace usage of 'cl with 'cl-lib

Fixes #165, wherein @dgutov rightly suggested switching over fully to `cl-lib`: this modernises the code, at the expense of a package dependency.

All tests are passing.

I haven't put in the legwork to make the non-snapshot travis build work properly, which would involve figuring out how to auto-install `cl-lib` there, but if this PR is generally acceptable, then I'll look into doing that.
Done.