docs: improvement of OS X installation guide

• BETTER Adds missing 'libffi' library and howto start redis server. Causing an exception when running pip install --process-dependency-links -e .[development]: 'ffi.h' file not found and 'sudo: service: command not found' when starting redis server (OS X Yosemite, 10.10).

Signed-off-by: Jochen Klein <j.klein@cern.ch>