Update CI due to Travis CI Pricing Model Changes

[frazierbaker]

Answer the following questions:

• what are you trying to do?
  Use continuous integration to help with reviews.

• what is the problem and how can it be recreated?
  Travis has a new pricing model, and we are out of credits. Try running any CI job.

• what scrimmage commit are you on? You can see this with git rev-parse HEAD
  Any commit will work to reproduce this issue.

• describe any changes you made to scrimmage.
  No changes to scrimmage.

• If you can recreate the issue using only plugins in the scrimmage repository, what mission file are you running?
  No mission being run, this is a CI issue.