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.

@MatthiasBrantner I have no strong opinion, just trying to make sure no one is blocked.