Tie `yasnippet` to it's current checkout

yasnippet installation is throwing an error on RHI's machine. Tie down
the checksum until I can debug and figure out the root cause.
Install `elpy` from el-get for python development.

Installing python from homebrew sets the Python Path correctly and
makes it accessible inside Emacs. Due to this, I can remove the explicit
python path setting in osx.el
=cider= master branch is broken

=cider-nrepl= causes continuous freeze-ups.

Tie cider down to it's 0.6.0 release and disable cider-nrepl middleware
until the problem fixed. Refer clojure-emacs/cider#534