Update routes to support Settings-Environment Variables

[lyalamanchili]

These endpoints are to support setting environment variables - https://docs.travis-ci.com/api#settings:-environment-variables

@pwmckenna Can you please merge the pull request?

[ljharb]

Seems legit

[ljharb]

@lyalamanchili altho tests are failing.

[pwmckenna]

lgtm. @ljharb you can merge/publish correct?

[ljharb]

Yup, I can! I'm going to let it sit in master for a bit before releasing, tho, and I'm hesitant to release before travis-ci can be fixed.

[travi]

i'm very confused by this addition but its quite likely that i'm missing something somewhat obvious.

when i attempt to even try getting env vars, with

travis.settings.env_vars.get((err, res) => {
    if (err) console.log(err)
    else console.log(res)
  })

i get the following error: Error: Couldn't find repository

from the docs, it looks like the repository_id needs to be provided as a query parameter, but i haven't figured out how to provide query parameters. am i overlooking how to supply query parameters?