This downloads a binary distribution from a Sage mirror for various systems supported by Travis CI: Mac OS X 10.10, 10.11, 10.12 (loaded with different XCode versions) and two versions of Ubuntu Linux on x86_64.
The scripts test that some optional packages can be installed and runs very limited doctesting.
On the free Travis CI infrastructure ("org"), there is a time limit of 1 hour per build. Downloading the binary distribution sometimes takes 30 minutes. Thus it is quite limited what can be tested.