Avoid travis build collision on S3 storage

[soffokl]

We are using S3 storage for sharing artifacts between build stages.
There could be collisions in Travis builds, since we are using the same bucket and location for all builds. Several parallel builds can affect each other.

We need to use $TRAVIS_BUILD_NUMBER to make our storage path independent.
And don't forget to clean S3 storage after build finished or failed. Travis support after_success and after_failure blocks.