CIRCLE: bump Openresty to 1.15.1 version.
CIRCLE: bump Openresty to 1.15.1 version.

Missed from commit `7910917d5388ad0c445837bf0b6dec6879bb3dc1` and
Circleci is not testing the profile target with the last openresty
version.

Signed-off-by: Eloy Coto <2a8acc28452926ceac4d9db555411743254cf5f9@gmail.com>

3scale/docker-gateway,3scale/docker-gateway