Use google.com probe instead of bsc.es

The main website of the BSC is failing every day around 3:00 AM for
almost one hour, so it is not a very good target. Instead, google.com is
used which should be more reliable. The same robots.txt path is fetched,
as it is smaller than the main page.

Reviewed-by: Aleix Roca Nonell <aleix.rocanonell@bsc.es>