switch to using the "deploy" step with a "script" provider

[mykmelez]

According to https://next.travis-ci.com/, Travis has shipped a "Custom" deployment provider:

A new deployment provider "Custom", which runs a single command for deployment has been deployed.

In the docs, they appear to call this the script provider:

https://docs.travis-ci.com/user/deployment/script

Oghliner currently uses a custom after_success step, which is supported by Travis:

https://docs.travis-ci.com/user/deployment/custom/

But the after_success command in Oghliner is pretty hairy, and it prevents users from using that step for anything else. It also doesn't play as nicely with the build lifecycle, which includes before_deploy and after_deploy steps that surround the deploy step:

https://docs.travis-ci.com/user/customizing-the-build/#The-Build-Lifecycle

So Oghliner should switch to using the deploy step with a script provider.