coveralls: Use gcov-5 since we build it using gcc-5
coveralls: Use gcov-5 since we build it using gcc-5

Reviewed-by: Emilia Käsper <emilia@openssl.org>

GH: #2003

openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl,openssl/openssl