Use Ruby 2.1.5

https://www.ruby-lang.org/en/news/2014/11/13/ruby-2-1-5-is-released/
Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Allow each app to edit these deploy instructions to fit their
  environments' needs (such as `run rake purge` to clear a cache)
  but maintain the same `./bin/deploy` interface and convention
  that is consistent with our many Middleman and Rails apps.
* Use the shebang line for POSIX sh.
* Default the argument to 'staging'
* Remove `set -e`.

The `set -e` functionality is not a 100% guarantee of expected semantics
(this is explored more by David Röthlisberger in [Shell scripts: Perils
of “set -e” for error handling][set-e]). In this case, the fact that the
script is two lines means that we can simply add a `&&` to the end of
the first line to get the desired semantics. Not using `set -e` means
that we have to understand how various commands can fail which,
honestly, is a feature.

[set-e]: http://david.rothlis.net/shell-set-e/