travis: remove the .travis.yml file

In build.git we haven't been using Travis for almost 3 years and even if
we would enable it again, it would probably not work since it's
configured to run a very old Ubuntu release. We also stopped using this
as a full regression build testing since a) our build time had increased
and b) Travis had become more popular that prolonged the build time c)
we often hit the run-time limit for free accounts.

Because of this and to avoid confusion we should remove the Travis
configuration.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jerome Forissier <jerome@forissier.org>