Generate random build id

@MatthiasBrantner I investigated if the client was doing something funky and the only thing I can see is that the job id might not be as unique as we think. The account always broke when we started to test another branch. If you look at https://github.com/28msec/28/blob/master/tests/sync.bats#L4 we always create random projects and these are run in parallel (https://github.com/28msec/28/blob/master/.travis.yml#L3). The account never broke.

This pull request generates a random build id.

This starts to be more and more confusing. At the beginning I wanted to make it easy to associate a deployment with a PR such that things like your cleanup don't happen because the names are descriptive. Then we went to build numbers which at least could be associated using Travis. Not it's going to completely random. I think Federico should spend some time to trace down the error before we completely randomize this.