Add devel spec for Emacs pretest 23.4.94

Adding a `devel` spec to the emacs recipe so that the latest pretest (24.3.94) for 24.4 can be installed, using same configuration as a HEAD build, some future tweaking might be required for `HEAD` and `devel` builds if the major versions differ. 

Command to install would then be:

`$ brew install emacs --devel <additional options, cocoa, srbg, etc...>
The parens aren't necessary here; this could also be `unless build.stable?`