New defcustom: elpy-show-installation-instructions

Prevents the *Elpy Installation* buffer from popping up.
Use line-number-at-pos instead of flymake-current-line-no ...

... since the latter has vanished as of

    commit 4cd39bde1061c55abb20a698d996637aaf705483
    Author: Leo Liu <sdl.web@gmail.com>
    Date:   2013-12-08 11:35:32 +0800

        Clean up flymake.el