travis: use commit from push/pr branch

- Use the branch from the "owner_name/repo_name" by setting up a
  temporary remote and checking out the TRAVIS_REPO_SLUG in build.git.
- Use the number of cores availabe to Travis.
- Increase depth to 10.
- Remove individual email address.
- Remove temporary build.git and toolchains.
- Remove unused "reference" cache.

Signed-off-by: Joakim Bech <joakim.bech@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>