Add Ert to development dependencies

Effectively superfluous, because ERT is built-in to Emacs 24, but serves
nicely as documentation of dependencies.
Add required Emacs version to dependencies

Prevents Flycheck from being `package-install`ed on Emacs 23.  The
`emacs` package is conveniently provided by `package.el` during
installation, with a version matching the one of the running Emacs.