Testing dpl in the context of Travis CI builds

It is possible to test new deployment provider or new functionality of dpl when it is used from the Travis CI build script.

To do so, add the following to your .travis.yml:

deploy:
  provider: X
  edge:
    source: myown/dpl
    branch: foo
  on:
    branch: TEST_BRANCH # or all_branches: true
  ⋮ # rest of provider X configuration

This builds the dpl gem on the VM from https://github.com/myown/dpl, the foo branch. Then it installs the locally built gem, and uses that to deploy.

Notice that this is not a merge commit, so it is important that when you are testing your PR, the branch foo is up-to-date with https://github.com/travis-ci/dpl/tree/master/.

When opening a PR, be sure to run at least one deployment with the new configuration, and provide a link to the build in the PR.