Only change to zsh if it is not the shell

`chsh` requires a password input, which we want to avoid where possible.
Don't use /etc/zshrc

* Using zshrc causes path_helper to override PATH from zshenv
* We set up rbenv among other things in zshenv

https://github.com/thoughtbot/dotfiles/pull/307
Changes heroku-config plugin url to use https

Many corporate firewalls block the git:// protocol.
Changing this URI to use https:// makes it usable for more people.
The other Git URIs are all using https://.
This was previously the only exception.
Make rbenv idempotent, install Ruby 2.1.4

* Don't repeat ourselves by using variables.
* Use more consistent variable naming and use of `local` in functions.
* Use `snake_case` for variable names and `ALLCAPS` for environment variables.
  https://github.com/thoughtbot/guides/tree/master/style#shell
* Use Bash double-bracket.
* Set current shell to newest Ruby version.
* Don't install `--pre` version of Bundler.

https://www.ruby-lang.org/en/news/2014/10/27/ruby-2-1-4-released/
Fix typo

https://github.com/thoughtbot/laptop/issues/319
Merge branch 'master' of https://github.com/thoughtbot/laptop