Migrate travis script to using sec-since-epoch as GUID

[ericmjonas]

Right now to create a GUID for interacting with AWS, we use a combination of travis environment variables as well as a random int generated by a python one-liner.

1. the one-liner is complicated because we have to account for python 2/3 differences
2. there's a non-zero chance it might fail

We should migrate to using seconds-since-epoch date +%s -- this will additionally make debugging easier.