Should talk about limiting Travis builds in travis-ci documentation

[Paul Beaudoin]

In our travis coverage, we should note ways to better leverage our common travis acct. Suggestion from @nodanaonlyzuul and @gkallenberg :

I think we can get more mileage out of the limited number of concurrent builds our Travis plan gives us by:

1. Doing fewer builds.
2. Making builds faster.
   Doing fewer builds is easy - by doing:

  only:
    - master
    - deployable-branch1
    - deployable-branch2

If you do this - TravisCI will not do builds for feature branches but WILL do builds for PRs.
We can make builds faster by asking travis to cache gems, node modules, (maybe even apt packages) https://docs.travis-ci.com/user/caching/.

[Matthew Koch]

This issue is quite old, over a year. Closing, with a tag of archived. @nonword FYI!