ci: retire travis configuration

ci: retire travis configuration

Travis CI was kind enough to offer free builds to open source for many
years (thanks!). Unfortunately, the inevitable abuse means that
travis-ci.org is ending on the 31st December 2020
(https://docs.travis-ci.com/user/migrate/open-source-repository-migration#frequently-asked-questions).
Current travis-ci.org users who wish to continue with Travis have to
create an account on travis-ci.com and will be put on a trial plan (see
https://blog.travis-ci.com/2020-11-02-travis-ci-new-billing ). This
becomes complicated for the fio project because although users are
gifted are 10,000 credits:
- If those credits are used for Linux builds you will be limited to 1000
  minutes worth of builds (each fio CI run does about 60 minutes of
  Linux builds)
- If those credits are used for macOS builds you will be limited to 200
  minutes worth of builds (each fio CI run does two macOS builds that
  take up about 25 minutes together)
- Even if you still have credit, the trial plan expires in a year
- An open source project can ask for more credit but it is a manual
  process that requires manually creating a support request
- The above means the initial credit would run out before the sixth CI
  run

Rather than going through a migration and the risk of using up all the
free credits let's retire Travis builds.

Signed-off-by: Sitsofe Wheeler <sitsofe@yahoo.com>