Use bin/setup from Travis

In order to test the executable documentation provided by bin/setup, we
should use `bin/setup` from CI.

I transitioned checks for gem presence to use `gem list --installed` and
to omit the `--no-document` flag. This flag should be decided by the
user's `.gemrc`.

Stop redirecting STDERR.

Expand grep flags and add --fixed-strings

The expanded flags tell me what this is doing. `--fixed-strings` gives a
tiny speed boost because we're not using regex here.
v.1.20.0
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'