Changes required by upstream renaming of nrepl to cider

`nrepl` has already been renamed to `cider` upstream, so all packages using `nrepl` need fixing up accordingly. This is an informed guess at what needs to be fixed in `midje-mode` -- please test and merge if possible. (`cider` is already in MELPA, but will not be added to Marmalade for a few days, so there's no need to publish a new version of `midje-mode` to Marmalade just yet.)

Cheers,

-Steve