Mkdir /usr/local for Homebrew if it doesn't exist
Use rbenv to change Ruby versions

* Use Homebrew consistently for OS programs.
* Use ruby-build to install Rubies.
* Include auto-switching of Ruby in `~/.zlogin`.
* Upgrade Rubygems after Ruby is installed. This is to help avoid common
  Bundler/Psych issues:
  https://github.com/tenderlove/psych/issues/66
* Use newest Bundler version in order to work with latest Rubygems.
* Use rbenv-gem-rehash so the shell automatically picks up binaries
  after installing gems with binaries.
Add new libraries to README
Alphabetize libraries in README
Force gcc-4.2 compiler when installing Ruby

Avoid errors like:

    st.c:520:35: error: implicit conversion loses integer precision:
    'st_index_t' (aka 'unsigned long') to 'int' [-Werror,-Wshorten-64-to-32]
    i = table->num_entries++; ~ ~~~~~~~~~~~~~~~~~~^~

https://github.com/sstephenson/ruby-build/issues/287
Install Hub gem

* Used by Suspenders' `--github organization/project` flag.
* `hub merge http://github.com/organization/project/pulls/123` for
  merging pull requests in open source projects.
* `hub browse` for opening the branch in a web browser.
* `hub pull-request` to create a pull request for the feature branch
  you're working in.
* Lots more.
Install The Silver Searcher instead of Ack

https://github.com/ggreer/the_silver_searcher

* ag searches faster than ack
* ag searches all files by default (but still ignores gitignored files).
  This removes the need for ack's --type-add= options.
Fix rbenv install

Error was:

    command not found: CC=gcc-4.2

* Set shell to Ruby 1.9.3 after installing it to make sure the following
 `gem` commands are using the correct Ruby.
* Use easier-to-read `--no-document` flag.