notifications.webhooks.on_start should accept values true and false

[thomasleveil]

It is documented that the webhooks option on_start can have true or false as value. From the travis-core project, it seems that on_start accepts the documented value true but also the undocumented value always.

Currently the linter warns for using values true or false and guides us to use either always, changed or never as in the SPEC.md file.

---

when testing

language: ruby
notifications:
  webhooks:
    urls: http://echo.jsontest.com
    on_start: true  # or false

actual result

in notifications.webhooks section: dropping on_start section: illegal value true

expected result

Hooray, your .travis.yml seems to be solid!