Use v1.6.1 of github.com/stretchr/testify in Travis
Use v1.6.1 of github.com/stretchr/testify in Travis

Later versions do not support Go 1.10-1.12, so use v1.6.1.

Change-Id: I10d77320784f9c04ea1a7d75c2e5e9e0549ecadb
Signed-off-by: Alexander Khabarov <410973c8966e1f78eb22e25cb3609e3bbce9769a@arm.com>

ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build,ARM-software/bob-build