.travis: Fix Python 3.5 build
.travis: Fix Python 3.5 build

Newer versions of pip are required to install recent versions of
cpp-coveralls. However, pip >= 21 is incompatible with Python 3.5.

Signed-off-by: Paul Chaignon <a027184a55211cd23e3f3094f1fdc728df5e0500@cilium.io>

iovisor/ubpf,iovisor/ubpf,rlane/ubpf,rlane/ubpf,iovisor/ubpf,iovisor/ubpf