travis-ci: Only rm old files from one build
travis-ci: Only rm old files from one build

This should eliminate simultaneous CI instances trying to remove the
same files, and erroring out, causing false failures on the CI.

Signed-off-by: Robin Getz <c66e5a86ef9317cf35cecb2ed063996b2100ed4b@analog.com>

analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio,analogdevicesinc/libiio