Add `bin/deploy` script

* Help avoid forgetting to run `rake db:migrate`.
* Help avoid forgetting to run `restart` after migrate.
* Use the shebang line for POSIX sh.
* Default the argument to 'staging'

We use it already in Cargo https://github.com/craftsmen/cargo/commit/99ec479a037ee53dae3eb47d3f06f02d015ecdd5