we were usng Codecov but weren't actually getting any notifications on pull requests.
Update codecov.yml
Removing `after_n_builds` as there is only 1 build
despite our attempts to fix it with the `after_n_builds` setting, it's still showing spurious fluctuations due to sometimes running elasticsearch tests and sometimes not
This should fix the issue that codecov reported a coverage percentage
which didn't take all builds into account (due to cancellation)
There are currently 29 travis builds so 10 should be a safe number
