Extract version values to global configuration flags.

[jverkoey]

This is intended to make it easier to understand what needs to change when a version of a given tool needs to be updated.

[jverkoey]

This is reopened from #9 but on the main repo so that Travis can run CI with the environment variables.

[romoore]

I don't see any "secure environment variables" in this change - can you clarify why you abandoned the earlier PR?

[jverkoey]

Encrypted environment variables are not available to pull requests from forks due to the security risk of exposing such information to unknown code.

https://docs.travis-ci.com/user/environment-variables/#defining-encrypted-variables-in-travisyml

[romoore]

Sorry, I don't think that helps. I still don't get how if it's "encrypted" it can be unencrypted when a PR comes in from the same repo but not from a fork. Does that mean that we'll NEVER be able to submit PRs to this repo from a fork?

[romoore]

Since your PR didn't modify any of those variables, it seems like Travis is broken - it can't run presubmits from forks which is the primary workflow for Material open source projects.

[jverkoey]

I've filed #12 to explore fixing this.