Change all `apt-get` references to `brew` EXCEPT postgres

[jrhorn424]

Related: #217

[MicFin]

@jrhorn424 I'd like to verify each of these instances of apt-get should be changed to brew:

1. libsass #217

   # LINUX ONLY
   sudo apt-get install libsass

2. Git

   ### LINUX ONLY
   Enter the command:
   `sudo apt-get install git`

3. Hub

   ####Linux ONLY
   Now you need to install some setup-tools:
   sudo apt-get install build-essential curl git python-setuptools ruby

4. rbenv dependency

   sudo apt-get install libffi-dev

[raq929]

OS X ONLY

Enter the command: brew install git

- Linux ONLY
-
-  Enter the command: sudo apt-get install git

[raq929]

5.Install ruby-build, a plugin for rbenv.

- # OSX ONLY
brew install ruby-build
- # LINUX ONLY
- git clone https://github.com/sstephenson/ruby-build.git ~/.rbenv/plugins/ruby-build

[raq929]

I have removed all of these except

# LINUX ONLY
sudo apt-get install build-essential curl git python-setuptools ruby

# LINUX ONLY
sudo apt-get install libffi-dev

and

    # LINUX ONLY
    sudo apt-get install postgresql libpq-dev

Which I think is appropriate. Please correct me if I'm wrong.