Configure Bundler for parallel gem installs

* Set it globally for OS X.
* Determine number of cores dynamically.
* Pick one number less than number of cores to avoid deadlock errors.
  http://archlever.blogspot.com/2013/09/lies-damned-lies-and-truths-backed-by.html
* Only install `--pre` for Bundler.
* Remove `pg` and `unicorn` gems as they will be installed during `bundle`
  for a Rails project.