Use conventional zsh dotfile names

* move rebenv initialization from .zlogin to .zshrc
* rbenv documentation recommends .zshrc
* less files for us to manage in our home directory

* use zshrc instead of zprofile
* zprofile is just an alternative for ksh fans [source](http://zsh.sourceforge.net/Intro/intro_3.html)
* Tim Pope recommends moving /etc/zshenv to /etc/zshrc [source](https://github.com/tpope/vim-rbenv)
Restart shell as login shell after rbenv

Move `exec $SHELL -l` line to spot recommended by [rbenv README][1].

Resolves [#77][2], where users would see errors like:

    Updating rubygems-update
    ERROR: While executing gem ... (Gem::FilePermissionError)
    You don't have write permissions into the /Library/Ruby/Gems/1.8
    directory.
    failed
    Installing critical Ruby gems for Rails development ...
    ERROR: While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-document
    failed
    Installing GitHub CLI client ...
    ERROR: While executing gem ... (OptionParser::InvalidOption)
    invalid option: --no-document
    failed

[1]: https://github.com/sstephenson/rbenv/
[2]: https://github.com/thoughtbot/laptop/issues/77
Don't open browser

* Opening the browser can surprise people.
* Use gerund in nearby command output.
Fix idempotence

Good catch by @gabebw:

https://github.com/thoughtbot/laptop/pull/88/files#r3739434
Remove unnecessary steps

* Permission fixing used to be necessary but Homebrew fixed this in this
  commit:
  https://github.com/mxcl/homebrew/commit/8aa4f9dd3bbd885350776171d3d9403d381e1a59
* Certain system libraries used to be necessary for Ruby 1.9.3 when we
  were on RVM, which enforced certain requirements:
  https://github.com/wayneeseguin/rvm/blob/master/scripts/functions/requirements/osx_brew#L93
* Improve printed output with newlines separating commands.
* Add more idempotent checks.
* Don't restart script in the middle of the run.
* Stick to `~/.zshrc`, not `~/.zshenv`.
* Finish script with preparation of `~/.zshrc` for thoughtbot/dotfiles'
  `install.sh` script.
* Add `fancy_echo`.
Ubuntu desktop does not come with aptitude or cURL

Since Ubuntu 10.10, aptitude has been left out of the default Ubuntu
desktop install. This commit changes the linux and linux-prerequisites
scripts to check if aptitude is installed: if it is the script continues
as normal; if it is not apt-get is used to install aptitude, and then
the script continues as normal.

This also updates README to use wget instead of cURL, and installs cURL
later.
Make sure line printed to ~/.zshrc is a comment
Merge branch 'master' of github.com:thoughtbot/laptop
Feature parity with recent OS X modifications

Includes:
* Removing SSH key generation
* Idempotency improvements
* Prep for dotfiles
* Install curl.
remove linux files