Update emacs setup docs to recommend flycheck instead of flymake #6681

Partially a reminder to myself, but if anybody else has already done it and thus can help, do let me know!

Currently, we recommend an emacs setup using flymake, pyflakes, and pep8 for syntax checking [1], but these are no longer developed (I noticed because I get warnings about syntax errors for metaclass=... arguments to class definitions). Instead, the recommendation is to use flycheck [2] with flake8 (which we're also using on travis). We should update the documentation (and I need to update my setup...).

[1] http://docs.astropy.org/en/latest/development/codeguide_emacs.html
[2] http://www.flycheck.org/en/latest/