fix short option version of "no-pull"

## Background
When running `centurion` with the `-n`, option, pull_image is still run.  It does not however, when passing "--no-pull".  This small change fixes the short option form.

Because of a quirk in how trollop is processing option names, the underscore causes the short option form to break.  I might submit an issue or PR to that repo, but not sure if it's still maintained :/

## Testing
* create a local docker image - `foo` - to deploy with centurion
* then under current master, reproduce the issue with: `bundle exec centurion -p foo -e development -a deploy -n`
* Note that doing this with `--no-pull` works
* checkout my branch, rerun previous steps