perf: Update README file with the recent changes.

* zone is now us-central1-f
* the ip address of the machine has also changed to 108.170.219.168.

BUG=None
TEST=None
R=jcgregorio@google.com

Review URL: https://codereview.chromium.org/654223010
Removed separate HTTP endpoint for counts of individual tests. Instead the existing endpoint for all counts is used and data are filtered on the frontend.

BUG=skia:

Review URL: https://codereview.chromium.org/680063002