Remove gnutls from the travis build
Remove gnutls from the travis build

Causing spurious fails of ssl-opt.sh. Likely a version issue. It would be
better to investigate the exact problem, and maybe adapt ssl-opt.sh to be
check for a minimum version just as compat.sh does, but this is a quick fix.

NXPmicro/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,NXPmicro/mbedtls,Mbed-TLS/mbedtls,NXPmicro/mbedtls,ARMmbed/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,ARMmbed/mbedtls,Mbed-TLS/mbedtls,Mbed-TLS/mbedtls